COVID-19 Vaccine Safety and Efficacy FAQs

July 15, 2021


How well do the COVID-19 vaccines work?

Fully vaccinated people are much less likely to get COVID-19 of any severity. The mRNA vaccines (Pfizer and Moderna) reduce the risk of symptomatic infection by more than 90% and appear to reduce the risk of asymptomatic infection by about 80%. Importantly, all three authorized vaccines provide near complete protection against developing severe infection that could lead to hospitalization or death. At a population level, the rising rates of vaccination across the country are thought to be an important reason why most regions are seeing large reductions in transmission. This has allowed more businesses and schools to fully open up and for the relaxation of masking, distancing and gathering limit requirements. At this time, the large majority of new COVID-19 cases are occurring in unvaccinated individuals.

Learn more about vaccine effectiveness from the Centers for Disease Control and Prevention (CDC) »

How long do COVID-19 vaccines last?

We do not yet know how long the COVID-19 vaccines work because it has been less than a year since the first large clinical trials started vaccinating people. The good news is that the vaccines appear to be working with very little waning effect over the eight to nine months that the trial participants have been followed. It is looking promising that for most people, strong protection may last for a year or more. It is possible that booster shots will be needed if new virus variants emerge and spread broadly, or if it turns out that the protection does significantly drop over time.

Why should I get vaccinated if COVID-19 infection is mild in most people and I am at low risk of complications?

First, vaccination will protect you. COVID-19 can cause severe illness, hospitalization, and death. The risk of these bad outcomes increases with some medical conditions and with age, particularly above age 50. It is true that in the majority of younger, healthy people COVID-19 usually causes mild symptoms, though severe infections can occur. Even in those who develop mild illness, the impact can be significant — missing up to 10 days of work or school due to mandatory isolation, reduced quality of life related to loss of taste or smell, and for approximately one in four people, symptoms may last for two or more months. Second, you receiving vaccine will protect others. When you protect yourself with the vaccine, you are much less likely to become infected and pass it on to somebody else, such as loved ones or coworkers who might be at higher risk of more severe illness. Also, by vaccinating a large percentage of our population, we slow virus transmission and reduce its ability to mutate into new and potentially more dangerous variants. We would also expect high vaccination rates to reduce case levels low enough for everybody to safely resume normal activities in just about every aspect of life.

Are COVID-19 vaccines recommended for those who have had prior COVID-19 infection?

Yes, reinfection can occur, and it is likely that the vaccines can reduce the risk of that happening. While there is evidence that prior infection can reduce the risk of younger people becoming re-infected by about 80%, we do not know how long that protection lasts. Research done at our own institution shows that some, but not all, people who had mild illness still had evidence of strong immunity 11 months after infection. Other studies have confirmed that people who have been previously infected have an excellent immune response to the vaccines. The CDC has more information why getting vaccinated is a safer way to build protection than getting infected.

Can people still get COVID-19 if they get a COVID-19 vaccine?

Yes, breakthrough infections are possible, but are rare. The COVID-19 vaccines can not cause COVID-19, and they are remarkably effective at lowering the risk of becoming infected; however even with greater than 90% effectiveness, they are not quite 100% perfect. The good news is that most breakthrough infections, when they occur, are mild, and very rarely lead to severe illness.

Are the COVID-19 vaccines effective versus the new variants?

Yes, the vaccines provide significant protection against the virus variants that have been circulating in the US. The vaccines may be slightly less effective against the beta, gamma, and delta variants, but studies so far suggest that the protection is robust, particularly against severe illness. The CDC and other agencies are constantly monitoring for the presence of new variants that can evade the immunity obtained from vaccines, and if that happens, slightly modified booster doses will be produced to protect against them.


Are the COVID-19 vaccines safe during pregnancy?

Limited data are available about the safety of COVID-19 vaccines in pregnancy. Based on how the vaccines work, they are unlikely to pose a risk to people who are pregnant or to the developing fetus. Pregnant people were not included in the original clinical trials but are included in new trials to help answer this question. As of early June 2021, more than 120,000 pregnant people have received a COVID-19 vaccine and, no serious safety concerns have yet been identified. Pregnant people are at increased risk for severe illness with COVID-19 and can receive a COVID-19 vaccine to reduce their risk. The mRNA vaccines have been shown to produce a strong immune response in pregnant people and the antibodies that are generated have been found in umbilical cord blood, suggesting it is possible they may protect the newborn as well, though this has yet to be confirmed. The CDC has additional information about COVID-19 vaccination while pregnant or breastfeeding and suggests that pregnant people facing this decision should consider their risk of exposure to COVID-19, the risks of severe illness, the known benefits of vaccination, and the available information about safety of vaccination during pregnancy.

What is known about the impact of COVID-19 vaccines on future fertility?

There is currently no evidence that any vaccine, including COVID-19 vaccines, cause either female or male fertility problems. There is also no biologically plausible reason to believe that COVID-19 vaccines should have any impact on the ability to conceive and bear children.

Will the mRNA vaccines affect my DNA?

No, mRNA COVID-19 vaccines do not alter your DNA. They deliver instructions (genetic material) to our cells to start building protection against the virus that causes COVID-19. However, the material never enters the nucleus of the cell, which is where our DNA sits. This means the genetic material in the vaccines cannot affect or interact with our DNA in any way.
Is it true that some people have died after receiving COVID-19 vaccines? Yes, but almost none of those have been due to the vaccine. As of early June, more than half the people in the United States (>172 million people) have received at least one dose of COVID-19 vaccine. On a typical day in the United States, an average of about 7800 people die from all causes. Statistically speaking, many of those people will have received a COVID-19 vaccine in the days or weeks prior to their death out of pure coincidence. To date, fewer than 10 deaths in the United States have been identified to be caused by a rare blood clotting disorder that may be vaccine-related.

How do we know the COVID-19 vaccines are safe?

The three COVID-19 vaccines authorized for use in the United States have demonstrated excellent safety profiles. Now that more than 300 million doses have already been given in the United States since December 2020, there is an abundance of information and a high degree of confidence about the safety of the vaccines. Information on safety comes from multiple sources, including the very large clinical trials that were conducted prior to Food and Drug Administration (FDA) emergency use authorization, and from monitoring of reported side effects through multipole studies, registries, and the Vaccine Adverse Event Reporting System (VAERS).

What are the adverse effects of the COVID-19 vaccines?

Mild local side effects such as arm soreness or redness are common. Up to half of people report fatigue, headache, muscle aches or chills that last one to two days, particularly after the second dose of a two-dose vaccine. Serious and life-threatening side effects have been very rare. Approximately two to five people per million who receive an mRNA vaccine (Pfizer or Moderna) may have a severe allergic reaction. This is not much different from many other medications and vaccines. Less than one per million recipients of the Johnson & Johnson/Janssen vaccine have experienced a serious, sometimes fatal, blood clotting disorder. Younger women appear to be the highest risk group at approximately seven per million. This is much lower than the estimated risk of blood clotting disorders that are attributed to some oral contraceptive medications (300-900 per million per year), though not all of these are severe. Recently, rare cases of inflammation of the heart muscle (myocarditis) or the outer lining of the heart (pericarditis) have been reported in adolescents and young adults after receiving a COVID-19 vaccine. There is ongoing research being conducted to understand the relationship between the vaccines and these rare conditions.

Read vaccine safety information from the CDC »

How much is known about long-term adverse effects of COVID-19 vaccines?

There is limited information on potential long-term adverse effects of COVID-19 vaccines because it has only been a little more than a year since the vaccines started clinical trial testing. Many millions of people are nearing or have surpassed six months since vaccination and to date no delayed vaccine adverse effects have been identified. Vaccine-related health problems occurring more than six weeks after vaccination are extremely unlikely following receipt of any vaccine, and there is no reason to believe that COVID-19 vaccines will lead to any increased risk.

Approval and production process

Why are the available COVID-19 vaccines not yet fully approved by the FDA?

Throughout the clinical trials and administration under emergency use authorization (EUA), the vaccines have proven to be extremely effective and safe. Across the country, there is high confidence that it is only a matter of time before the FDA grants full approval. As of early July 2021, COVID-19 vaccines have been administered to over 180 million Americans — more than half the country — and the number increases each day. The currently authorized vaccines have very similar safety profiles to other vaccines and are more effective than most other vaccines for adults.

The COVID-19 vaccines were developed very quickly. How do I know they are safe?

The COVID-19 vaccines were developed in record time out of necessity in direct response to the worst pandemic the world has seen in over a century. The relatively short time period to launch safe and effective vaccines, compared to the usual slow and deliberate process, was accomplished by a combination of advanced technology, global information sharing on the details of the virus genetic sequences, very high levels of COVID-19 transmission that allowed for short follow-up times needed to reach the required number of cases between the vaccine and placebo recipients in clinical trials, and massive amounts of funding invested by the pharmaceutical industry and governments that allowed enrollment of very high numbers of trial participants. The clinical trials were conducted with the same level of scientific review and oversight as other vaccine studies, and the data were similarly carefully monitored by safety committees. The FDA followed a rigorous process to review the available data before granting emergency use authorization.

What are the ingredients in the COVID-19 vaccines?

Vaccine ingredients can vary by manufacturer. In general, the mRNA vaccines contain messenger RNA (the component that tells our cells to make spike protein that stimulates the immune response), surrounded by a lipid (fatty) coating, and buffers that maintain the stability of the product. The Johnson & Johnson/Janssen vaccine contains an adenovirus that does not cause human disease but carries the genetic material that tells our cells to make spike protein, as well as buffers that stabilize the product.

Learn more about the ingredients in authorized COVID-19 vaccines