COVID-19 Vaccine Questions and Answers

Operation Warp Speed?

What is Operation Warp Speed?

Operation Warp Speed (OWS) is coordinating the development, manufacturing, and distribution of COVID-19 vaccines, therapeutics, and diagnostics (collectively known as countermeasures). A central aim of OWS is to produce and deliver enough vaccine doses sufficient to immunize the American public.

OWS is a partnership among components of the Department of Health and Human Services (HHS), including the Centers for Disease Control and Prevention (CDC), the National Institutes of Health (NIH), and the Biomedical Advanced Research and Development Authority (BARDA), and the Department of Defense (DoD). OWS engages with private firms and other federal agencies, including the Department of Agriculture, the Department of Energy, and the Department of Veterans Affairs. It is coordinating existing HHS-wide efforts, including NIH's Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV) partnership, NIH's Rapid Acceleration of Diagnostics (RADx) initiative, and work by BARDA.

The OWS effort is allowing countermeasures such as a vaccine to be delivered more rapidly while adhering to the highest standards for safety and efficacy. Rather than eliminating steps from the traditional vaccine development timeline, steps are occurring simultaneously, such as starting manufacturing of the vaccine at industrial scale well before the demonstration of vaccine efficacy and safety. This increases resource use without sacrificing safety.

View the HHS video COVID-19 Vaccine Update | How Vaccines are Developed for more information.

What is NIAID’s role in Operation Warp Speed’s vaccine effort?

NIAID is part of NIH, the nation’s medical research agency. NIAID established the COVID-19 Prevention Network (CoVPN) by merging and expanding on multiple existing NIAID-funded clinical trials networks. CoVPN sites are participating in the OWS-supported vaccine clinical trials and providing many of the clinical trial sites. In addition, CoVPN investigators serve as co-principal investigators on the studies. NIH experts, through the Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV) public-private partnership, have advised on the protocol design for each of the trials. Additionally, NIAID experts are members of each trial oversight group—along with the sponsor and BARDA— that receives recommendations from the vaccine trials’ Data and Safety Monitoring Board (DSMB).

Although NIAID and BARDA are providing funding support to the vaccine trials, NIAID and the U.S. government do not serve as the “regulatory sponsor” of the OWS-supported vaccine trials. Rather, the vaccine companies developing each vaccine have this role. The regulatory sponsor has legal responsibility for conduct of the trial and manages the trial data.

COVID-19 Vaccine Clinical Trials

What is a Phase 3 vaccine clinical trial?

A Phase 3 trial of an investigational vaccine enrolls thousands of people to evaluate if the vaccine is safe and can effectively prevent symptomatic COVID-19 disease. All vaccine candidates being tested in OWS-supported Phase 3 clinical trials have been previously tested in early-stage clinical trials that showed they were well tolerated and elicited an immune response in adult volunteers. Participants in Phase 3 clinical trials are assigned randomly to receive either the investigational vaccine or a placebo. The trials are double-blind, meaning neither the trial investigators nor the participants know who received the vaccine candidate. Investigators evaluate if the vaccine works by comparing the number of cases of symptomatic COVID-19 in the vaccine group versus the placebo group. Participants are monitored throughout the trial for safety.

For more information, view the NIH video, What is a Phase 3 Clinical Trial for a Vaccine Candidate?

What types of COVID-19 vaccines are being evaluated in large-scale trials in the U.S.?

Please visit the NIH ACTIV vaccines page for a summary of Operation Warp Speed-supported clinical trials of COVID-19 vaccine candidates.

Viral Vector-Based Vaccines

The candidate vaccines developed by AstraZeneca and Oxford (AZD1222) and the Janssen Pharmaceutical Companies of Johnson & Johnson (JNJ-7843672 or Ad.26.COV2.S) are viral vector-based vaccines. They use a safe virus to deliver the genetic code (DNA) of the SARS-CoV-2 spike protein to human cells so that the cells can make the protein. JNJ-7843672 uses a human adenovirus to deliver the code for the SARS-CoV-2 spike protein. Adenoviruses are a group of viruses that cause the common cold. However, the adenovirus vector used in the vaccine candidate has been modified so that it can no longer replicate in humans and cause disease. Janssen uses the same vector in the first dose of its prime-boost vaccine regimen against Ebola virus disease (Ad26.ZEBOV and MVA-BN-Filo), which is licensed for use in Europe. AZD1222 uses a non-replicating chimpanzee adenovirus to deliver the code for a SARS-CoV-2 spike protein. The technology is based on a vaccine that Oxford was previously developing for Middle East respiratory syndrome coronavirus (MERS-CoV).

mRNA Vaccines

The vaccine candidates developed by Pfizer and NIH/Moderna are mRNA (messenger ribonucleic acid) vaccines. When the genetic code (DNA) of a protein is delivered to a cell, it is decoded by RNA into readable instructions. mRNA vaccines skip this step by providing directly to cells the readable instructions on how to make the SARS-CoV-2 spike protein. Lipid nanoparticles deliver the mRNA to the cell’s cytoplasm. The mRNA from a COVID-19 vaccine never enter the nucleus of the cell, which is where our DNA are kept. This means the mRNA does not alter or interact with our DNA in any way, and is eventually degraded inside the cell as part of the natural mRNA lifecycle.

Protein Subunit Vaccine

The vaccine candidate developed by Novavax, Inc., (NVX CoV2373) is a protein subunit vaccine that contains a stabilized form of the SARS-CoV-2 spike protein using the company’s recombinant protein nanoparticle technology. Other COVID-19 vaccines under development are nucleic acid vaccines that deliver instructions for cells to make the protein, whereas protein subunit vaccines deliver the protein directly.

How are the trials of COVID-19 vaccines aligned?

A clinical trial protocol is developed by the trial sponsor and stipulates why and how the trial is being conducted. The protocol covers the scientific rationale for the trial, its key objectives, the design and methodology, which populations can and cannot enroll, when and how efficacy is analyzed, how safety is monitored, statistical considerations and other issues.

All OWS Phase 3 trials are randomized and placebo-controlled, which is the gold standard for rigorous clinical research. NIH experts, through the Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV) public-private partnership, helped to ensure the protocols of all OWS-supported Phase 3 trials of investigational vaccines use the same laboratory tests and are designed to evaluate a similar primary endpoint: whether the vaccine can prevent symptomatic COVID-19. This approach enables transparent evaluation of the relative performance of each vaccine approach across trials. The trials also share a common Data and Safety Monitoring Board, an independent body which is there to oversee study scientific rigor and volunteer safety.

More information on the harmonized approach to clinical trials of COVID-19 vaccines is covered by NIH experts in a May 29, 2020, Policy Forum in the journal Science, titled “A strategic approach to COVID-19 vaccine R&D.”

How long will the trials take to complete?

Each trial is designed to follow participants and collect safety and immunogenicity data for about two years. However, a trial can be temporarily paused or stopped due to safety issues, futility or strong evidence of efficacy. It is difficult to predict exactly how long it will take to generate enough data to determine the safety and efficacy of various vaccine candidates because that depends on the actual incidence of SARS-CoV-2, the virus that causes COVID-19. The trials are event-driven (case-driven), meaning that statisticians have estimated that a certain number of cases of COVID-19 among trial participants (around 150-160) must be observed for a high probability of detecting a percent reduction in disease incidence in the vaccinated group compared to the placebo group that is not due to chance. The primary analysis occurs once this threshold of cases has been observed. The trial protocols also stipulate if and when the Data and Safety Monitoring Board (DSMB) can perform interim analyses before reaching the point for primary analysis.

Why can’t certain populations like children and pregnant women enroll in the current COVID-19 trials?

It is common practice to first evaluate the safety and efficacy of an investigational vaccine in adult volunteers before evaluating in children and pregnant women. However, OWS recognizes the importance of vaccine-induced protection in all populations, including children and pregnant women. OWS is accelerating preclinical assessments of potential developmental and reproductive toxicity to anticipate any safety concerns, as recommended per guidance from the Food and Drug Administration. Assuming those assessments show no safety concerns, OWS intends to conduct safety and immunogenicity studies in pregnant women and children, in partnership with industry. The immune responses in those studies would be compared to the immune responses from Phase 3 trials of non-pregnant adults (known as “immunobridging”).

How is NIAID working to ensure diversity among volunteers in the vaccine clinical trials? Why is enrolling diverse groups important?

Long-standing systemic health and social inequities have put many people from racial and ethnic minority groups at increased risk of getting sick and dying from COVID-19. Race and ethnicity are risk markers for other underlying conditions that impact health — including socioeconomic status, access to health care, and increased exposure to the virus due to occupation (e.g., frontline, essential, and critical infrastructure workers). According to the CDC, people who are Black or African American, Hispanic or Latino, American Indian or Alaska Native and Asian are more likely to die from COVID-19. Black and African American people are 3.7 times more likely to be hospitalized for COVID-19 as compared to white people. Hispanic or Latino people are 4.1 times more likely to be hospitalized, American Indian or Alaska Native people are 4 times as likely to be hospitalized, and Asian people are 1.2 times as likely to be hospitalized.

It is important that U.S. clinical trials enroll a volunteer population that at least reflects the diversity of the U.S. population, or more ideally—reflects the population of those at increased risk of COVID-19.

The NIAID-funded COVID-19 Prevention Network (CoVPN) is working with stakeholders to reach priority populations, including Native Americans, Black Americans (including African Americans), the Latinx community, people who are at higher risk of exposure to SARS-CoV-2 infection due to occupation, people with pre-existing health conditions, people living in assisted living facilities and communities experiencing health disparities.

The NIH Community Engagement Alliance (CEAL) Against COVID-19 Disparities is conducting outreach and seeking input from communities to raise awareness about COVID-19 and to address misinformation and mistrust about the pandemic and efforts to combat it. CEAL also is working to ensure that COVID-19 prevention and treatment clinical trials include racially and ethnically diverse communities most affected by the pandemic.

What is a Data and Safety Monitoring Board (DSMB)? What is unique about the Operation Warp Speed vaccine DSMB?

A DSMB is an independent group of experts from various relevant disciplines who oversee and monitor clinical trials to ensure participant safety and the validity and integrity of the data. OWS-supported Phase 3 vaccine trials are overseen by a common DSMB (meaning the same DSMB for multiple trials). The OWS DSMB includes members with expertise in ethics, statistics, vaccine development, patient care and clinical trials. Their role is to advise a group composed of the study sponsor, NIAID and BARDA.

The DSMB members meet before each study starts to review the protocol and statistical monitoring plan. After that point, the DSMB meets according to the schedule laid out in the protocol. At all regularly scheduled meetings, there is an open and closed session. The DSMB reviews interim and final data on safety and efficacy at prespecified points throughout a study and makes recommendations to the sponsor regarding whether a protocol should be amended, or the study should proceed or be paused or terminated.

What is NIAID’s role in relation to the DSMB?

NIAID formed the OWS DSMB and invited the members. A NIAID employee serves as the executive secretary.

The DSMB provides recommendations to an oversight group composed of the study sponsor, NIAID and BARDA. If there is no consensus among the oversight group, which is rare, the ultimate decision belongs to the sponsor. NIAID Director Anthony S. Fauci, M.D., serves as the designated senior representative of the United States government for the oversight group. He does not participate in the DSMB process of developing recommendations regarding a clinical trial.

Who is part of the OWS DSMB?

The DSMB is composed of a group of experts from various relevant disciplines. The names of DSMB members are confidential to protect the integrity of the process. The only time DSMB members are named is when a study has concluded and then, only with their express permission, they may be named in the acknowledgments section of a published manuscript for the study on which they served. Each member receives a modest honorarium for each meeting and is responsible for disclosing any potential conflict of interest.

Are vaccine trials safe?

Researchers first evaluate experimental vaccines in the laboratory and animal models. If a vaccine candidate is safe and appears promising in these preclinical experiments, it may go on to be carefully tested in people. Experimental vaccines undergo several phases of clinical testing to establish their safety and efficacy. After a vaccine is licensed or issued an Emergency Use Authorization, FDA, CDC, and other federal agencies continue to monitor its safety.

Developing safe vaccines and ensuring the safety of the volunteers who participate in vaccine clinical trials are of the utmost importance. By their nature, all clinical trials involve the assumption of some level of risk. The risk assumed by well-informed adult volunteers could have enormous benefit to society by accelerating the development of safe and effective vaccines during this pandemic. All vaccine candidates being tested in OWS-supported Phase 3 clinical trials have been previously tested in early-stage clinical trials in which they were found to be well tolerated and prompted an immune response in adult volunteers.

People can say yes or no when invited to join a study of a COVID-19 vaccine candidate. All study volunteers must go through a process called informed consent that ensures they understand potential risks and benefits of being in a study. They also may leave a study at any time. Study sites make every effort to ensure that people understand the study fully before they decide whether to join. The trials adhere to U.S. federal regulations on research, as well as international ethical standards.

Clinical trial participants are very closely monitored for side effects attributable to the experimental vaccine. Most commonly, vaccine side effects are temporary and may include soreness or redness at the injection site, and less commonly, fever. Very rarely, a person may experience a serious adverse event. Clinical trial participants also are counseled on steps to take to minimize the possibility of contracting COVID-19 in their communities.

Visit the COVID-19 Prevention Network Frequently Asked Questions page for more information.

What is a clinical trial pause?

It is not unusual for clinical trials to pause enrollment when a safety event occurs, which has happened with a few clinical trials of vaccines and therapeutics for COVID-19.  Study protocols delineate the type of safety events that must lead to an extra level of review and a halt or pause on new enrollment. This illustrates the extent to which safeguards for volunteer safety are built into clinical trials. A pause in a trial is a sign that the system is working. A study sponsor decides to initiate a pause.

For more information, view the BARDA video, What does it mean when a clinical trial is paused?

What is a clinical hold?

Another type of study halt is known as a clinical hold. The Food and Drug Administration (FDA) oversees the conduct of clinical trials in the U.S. and can issue a clinical hold, requiring the sponsor to respond to various questions before the trial can resume.

What happens after a Phase 3 clinical trial establishes a vaccine is safe and effective?

A clinical trial must first establish a vaccine candidate’s safety and efficacy before it can be made widely available. Any COVID-19 vaccine candidate will follow this process before people can receive the vaccine outside of a clinical trial:

  1. Safety and Efficacy Established
  2. FDA Reviews and Authorizes
  3. CDC Issues Recommendations on Vaccine Use
  4. Vaccine is Distributed

Safety and Efficacy Established

The Data and Safety Monitoring Board (DSMB) reviews trial data and informs the sponsor, NIH and BARDA that the trial has reached a predefined “stopping point” for efficacy. This means there is enough evidence to show that the vaccine can effectively prevent symptomatic COVID-19 in participants. Even if a “stopping point” is reached, the trial staff will continue to follow participants and may remain blinded. In addition, the trials are still allowing adequate time to ensure enough safety data is collected before a vaccine is widely distributed. The DSMB will play a key role in advising how a trial should proceed ethically once a stopping point is reached.

FDA Reviews and Authorizes

FDA's Center for Biologics Evaluation and Research (CBER) is responsible for regulating vaccines in the United States. If successful, the completion of all three phases of clinical development can be followed by a request for an Emergency Use Authorization (EUA) or the submission of a Biologics License Application (BLA).

The CBER Vaccines and Related Biological Products Advisory Committee (VRBPAC) met in October 2020 to discuss, in general, the development, authorization and/or licensure of vaccines to prevent COVID-19. This non-FDA expert committee (scientists, physicians, biostatisticians, and a consumer representative) provides advice to the FDA regarding the safety and efficacy of the vaccine for the proposed indication. VRBPAC is reviewing requests for EUA and license applications for all COVID-19 vaccines. See FDA’s Emergency Use Authorization for Vaccines Explained for more information.

CDC Issues Recommendations on Vaccine Use

CDC makes COVID-19 vaccination recommendations for the United States based on input from the Advisory Committee on Immunization Practices (ACIP). ACIP is a federal advisory committee made of up of medical and public health experts who develop recommendations on the use of vaccines in the U.S. public.

If the Food and Drug Administration (FDA) authorizes or approves a COVID-19 vaccine, ACIP convenes to review all available data about that vaccine. From these data, ACIP will then vote on whether to recommend the vaccine and for which populations. ACIP’s recommendations also include guidance on who should receive COVID-19 vaccines if supply is limited. Recommendations must go to the director of CDC for approval before becoming official CDC policy.

For more information, please see How CDC Is Making COVID-19 Vaccine Recommendations.

Vaccine is Distributed

The CDC, per initial guidance from ACIP, has outlined priority populations who will receive the vaccine first. Generally, it will take several months before COVID-19 vaccines are widely available to the general public.

The Department of Defense is helping to coordinate vaccine distribution logistics in the United States. While the Department of Defense is overseeing this process, it is important to note that military personnel will not be physically vaccinating people in the U.S., although some states may opt to use their National Guard medical corps for assistance.

The CDC notes that the federal government will oversee a centralized system to order, distribute, and track COVID-19 vaccines. All vaccines will be ordered through CDC. Vaccine providers will receive vaccines from CDC’s centralized distributor or directly from a vaccine manufacturer. For more information, see Frequently Asked Questions about COVID-19 Vaccination.

How did NIAID research contribute to the early development of COVID-19 vaccines now in late-stage clinical testing?

Building on previous research on SARS and MERS, NIAID scientists and grantees were positioned to rapidly develop COVID-19 vaccines, therapeutics and diagnostics. These projects include conducting basic research to understand how the virus infects cells and causes disease, and what interventions can prevent and stop the spread of disease.

Scientists at NIAID’s Vaccine Research Center in Bethesda, Maryland have spent years researching the structure and function of coronaviruses to make highly targeted vaccines. Together with their academic collaborators, their pivotal work prior to the COVID-19 pandemic revealed that a stabilized version of the spike protein found on the surface of all coronaviruses can be a key target for vaccines, therapeutics and diagnostics. All coronavirus particles are spherical and have mushroom-shaped proteins called spikes protruding from their surface, giving the particles a crown-like appearance. The spike binds and fuses to human cells, allowing the virus to gain entry. However, the spike undergoes a massive rearrangement as it fuses the virus and cell membranes. VRC researchers and their colleagues (Andrew Ward at Scripps and Jason McLellan at Dartmouth) solved the spike structure. VRC and Dr. McLellan (now at University of Texas, Austin) found that the spike stabilized in its prefusion conformation is more likely to preserve targets for infection-blocking antibodies induced by a vaccine. Thanks to years of prior research, once the genome of a SARS-CoV-2 virus isolate was shared by researchers in China in early 2020, NIAID VRC scientists and their UT collaborators were able to quickly confirm that the SARS-CoV-2 stabilized prefusion spike protein would be a candidate vaccine antigen. Their research revealing the atomic-level structure of the SARS-CoV-2 spike protein is supporting discovery of antiviral therapeutics in addition to precision vaccine design.

NIAID scientists based at Rocky Mountain Laboratories in Hamilton, Montana, also have been collaborating with Oxford University investigators to conduct preclinical studies on Oxford’s chimpanzee adenovirus-vectored vaccine candidate against MERS-CoV and quickly transitioned to preclinical studies of Oxford’s candidate for SARS-CoV-2 (AZD1222), now licensed to AstraZeneca for further development.

How can I volunteer for a COVID-19 vaccine clinical trial?

Those interested in participating in a COVID-19 vaccine clinical trial can visit

Where can I find more information about participating in a COVID-19 treatment clinical trial?

Those interested in participating in COVID-19 clinical trials can visit

When will participants in the Moderna and Pfizer/BioNTech vaccine trials who received a placebo gain access to the vaccine?

The study sponsors (Moderna and Pfizer/BioNTech), in consultation with the FDA, are establishing plans to offer the vaccines to participants who were randomized to receive placebo as part of the trial. Please contact the study sponsors directly for more information.

Will placebo-controlled trials of other COVID-19 vaccine candidates continue once the Moderna and Pfizer vaccines receive EUA? Will new placebo-controlled trials begin?

Additional Operation Warp Speed-supported placebo-controlled trials initiated before the Moderna and Pfizer/BioNTech vaccines were made available are still underway. The trials are evaluating investigational vaccines developed by AstraZeneca and the Janssen Pharmaceutical Companies of Johnson & Johnson.

Another Operation Warp Speed-supported trial evaluating an investigational COVID-19 vaccine developed by the biotechnology company Novavax, Inc., began in late December 2020. It is important to continue to conduct rigorous trials of multiple types of COVID-19 vaccines to ensure we have an array of options. One-size-fits-all preventions do not always provide the best fit for everyone. COVID-19 vaccines initially available to certain priority populations are being authorized by the Food and Drug Administration under a mechanism known as Emergency Use Authorization (EUA). The issuance of an EUA is different than an FDA approval (licensure) of a vaccine. A vaccine available under emergency use authorization is still considered investigational. A placebo-controlled trial remains appropriate until a COVID-19 vaccine is approved for use and widely available in the U.S. At that point, a head-to-head clinical trial design comparing the investigational vaccine to the licensed vaccine would be appropriate.

General Questions About COVID-19 Vaccines

When will a COVID-19 vaccine be widely available in the U.S.?

Operation Warp Speed made initial doses of safe and effective COVID-19 vaccines available in December 2020 following Emergency Use Authorizations granted by the FDA. However, it likely will take several months before vaccines are widely available to the general public, based on the priority populations recommended by the CDC.

When will the COVID-19 pandemic end?

Proven public health measures can help mitigate the COVID-19 pandemic, including face coverings (masks), physical distancing, practicing good hand hygiene, limiting crowds and gatherings, and prompt testing (and proper isolation and quarantine of infected individuals and contacts). However, safe and effective medical countermeasures for COVID-19 are urgently needed to save lives and ultimately control the pandemic.  “Low tech” tools such as mask wearing will still be needed after a vaccine is initially available because it will take at least several months before enough people are vaccinated to confer herd immunity in the general population. Combination COVID-19 prevention measures will remain essential.

Why do some COVID-19 vaccines require two shots?

Preclinical and early-stage clinical trials help determine the most effective vaccine dose and regimen for inducing a robust and durable (lasting) immune response. For most of the candidate COVID-19 vaccines, these studies showed that two doses separated by 3 to 4 weeks are better than one. Many routine vaccinations in the United States, such as HepB (for hepatitis B), inactivated poliovirus (for polio), and DTaP (for diptheria, tetanus and whopping cough) are administered as multiple shot regimens.

Moderna and Pfizer have reported 95% efficacy for their COVID-19 vaccines. What does this mean?

A COVID-19 vaccine that has 95% efficacy means that that the incidence of symptomatic COVID-19 was 95% lower in those participants who were vaccinated as part of a clinical trial as compared to those receiving placebo. Vaccine efficacy refers to the proportionate reduction in cases among vaccinated individuals under ideal circumstances, such as in a clinical trial. The term vaccine effectiveness is a measure of vaccine protection in less controlled, real-world conditions. It will take time to learn the effectiveness of COVID-19 vaccines.

Will a COVID-19 vaccine prevent against severe disease?

Data from Phase 3 clinical trials of the Pfizer/BioNTech and Moderna COVID-19 vaccines indicate that the vaccines are highly effective in preventing severe disease. As part of the Phase 3 clinical trials of COVID-19 vaccines, investigators monitor daily anyone with confirmed infection to track symptoms so that participants who may progress to more serious illness (such as those requiring hospitalization) can be categorized and followed. Therefore, it is possible to compare rates of progression to severe COVID-19 among vaccine and placebo recipients.

Will COVID-19 vaccines prevent transmission of the virus that causes COVID-19?

Initial data shows that the COVID-19 vaccine candidates developed by Pfizer and Moderna/NIH can prevent people from developing symptomatic COVID-19, including serious illness. As these vaccines are authorized for use in the U.S., getting vaccinated yourself may also protect people around you, particularly people at increased risk for severe illness from COVID-19; however, more data is needed to understand how well the vaccines can keep people from spreading the virus that causes COVID-19.

As additional data from current Phase 3 trials become available, we will obtain a better sense of the vaccines’ impact on seroconversion, asymptomatic SARS-CoV-2 infection and their ability to prevent vaccinated individuals from transmitting SARS-CoV-2. Additionally, more extensive testing of asymptomatic individuals will help determine whether vaccination is having an impact on transmission.

Is natural immunity to COVID-19 better than vaccine-induced immunity?

At this time, experts do not know how long someone is protected from getting sick again after recovering from COVID-19. The immunity someone gains from having an infection varies from person to person. Although evidence shows that infection with other viruses usually leads to strong and durable natural immunity, this may not be the case for COVID-19, and a vaccine may provide a better immune response. Both natural immunity and vaccine-induced immunity are important aspects of COVID-19 that experts are trying to learn more about. Importantly, COVID-19 vaccination helps protect you by creating an immune response without having to experience sickness.

If someone has had SARS-CoV-2 infection previously, will they still need to get a COVID-19 vaccine?

CDC’s Facts about COVID-19 Vaccines notes that people who have gotten sick with COVID-19 may still benefit from getting vaccinated. Due to the severe health risks associated with COVID-19 and the fact that re-infection with COVID-19 is possible, people may be advised to get a COVID-19 vaccine even if they have been sick with COVID-19 before. At this time, experts do not know how long someone is protected from getting sick again after recovering from COVID-19. The immunity someone gains from having an infection, called natural immunity, varies from person to person. Some early evidence suggests natural immunity may not last very long.

We won’t know how long immunity produced by vaccination lasts until we have a vaccine and more data on how well it works. Both natural immunity and vaccine-induced immunity are important aspects of COVID-19 that experts are trying to learn more about, and CDC will keep the public informed as new evidence becomes available.

Is it safe for people to get a COVID-19 vaccine if they previously had COVID-19? Will tests for prior infection be required before vaccination in case someone was unknowingly infected?

According to the CDC, previous SARS-CoV-2 infection, whether symptomatic or asymptomatic, is not considered a contraindication to vaccination and serologic testing for SARS-CoV-2 antibodies is not recommended prior to vaccination. See How CDC Is Making COVID-19 Vaccine Recommendations for more information.

Will people who have already had COVID-19 be de-prioritized for receiving the vaccine initially?

Given the limited number of vaccine doses that will be initially available, the CDC has provided initial guidance on which populations should receive authorized COVID-19 vaccines first. The CDC has also laid out clinical considerations for Phase 1a vaccine recipients (healthcare personnel and long-term care facility residents) here. Within this guidance, the CDC notes that, previous SARS-CoV-2 infection, whether symptomatic or asymptomatic, is not considered a contraindication to vaccination and serologic testing for SARS-CoV-2 antibodies is not recommended prior to vaccination. If sub-prioritization of vaccine doses among healthcare workers is necessary, CDC notes that those with documented acute SARS-CoV-2 infection in the preceding 90 days may choose to delay vaccination until near the end of the 90 day period in order to facilitate vaccination of those HCP who remain susceptible to infection, as current evidence suggests reinfection is uncommon during this period after initial infection.

What is an mRNA vaccine and how does it work?

mRNA (messenger ribonucleic acid) vaccines are a new type of vaccine, although the technology has been researched by scientists for many years. Vaccines work by introducing a weakened or inactivated virus, or a virus protein, to the body to induce an immune response. mRNA vaccines provide instructions to human cells on how to make the protein. Our immune systems recognize that the protein doesn’t belong there and begin building an immune response, like what happens in natural infection against COVID-19. See the CDC’s page on Understanding mRNA COVID-19 Vaccines for more information.

Does an mRNA vaccine interact with our DNA?

No. As the CDC explains, and as NIH scientists can confirm, mRNA is not able to alter or modify a person’s genetic makeup (DNA). The mRNA from a COVID-19 vaccine never enter the nucleus of the cell, which is where our DNA are kept. This means the mRNA does not alter or interact with our DNA in any way. Instead, COVID-19 vaccines that use mRNA work with the body’s natural defenses to safely develop protection (immunity) to disease. See the CDC’s page on Understanding mRNA COVID-19 Vaccines for more information.

Will a vaccine be safe for people who have conditions like diabetes, a rare disease, neurological conditions, or are immune compromised?

Phase 3 clinical trials of investigational COVID-19 vaccines have enrolled participants with various comorbidities. Information about vaccine performance in individuals with specific conditions will be available as more data is collected and analyzed. The CDC will provide recommendations on the use of COVID-19 vaccines and will note if the vaccine is contraindicated for any specific populations.

Can influenza and the COVID-19 vaccine be combined to form a single-dose annual vaccine in the long run?

It is too early to know how long protection with a COVID-19 vaccine will last, nor whether the influenza vaccine would be compatible with COVID-19 vaccine components in a vaccine. Therefore, currently, NIAID cannot say whether an annual vaccination will be required.

Some vaccines, like those for chicken pox and measles, offer lifelong protection after a single vaccination regimen. How often will COVID-19 vaccinations be required?

We won’t know how long immunity produced by COVID-19 vaccination lasts until we gather more data.

How do scientists figure out how long a vaccine will be protective? Is this measured by how long antibodies to the virus last in our blood?

Vaccines prompt the immune system to make antibodies, which are the protein molecules in your blood that are made by infection-fighting white blood cells called B cells and by specialized plasma cells in bone marrow. Vaccines may also lead to the expansion of infection-fighting T cells that can recognize virus-infected cells and rapidly clear them. However, even when antibodies are no longer detectable in a person’s blood, it is likely that they still have immunological memory in the form of memory T and B cells, which can respond more rapidly to new infections.

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