COVID-19 Vaccines Safety and Effectiveness

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An MSK employee after receiving a COVID-19 vaccine

Large studies have found that the COVID-19 vaccines are highly effective and safe.

Memorial Sloan Kettering has begun the critically important mission to administer COVID-19 vaccines. We want to be sure you understand their safety, their effectiveness, and how they work.

Both the Pfizer-BioNTech and Moderna vaccines have been given emergency use authorization by the Food and Drug Administration (FDA). Large studies involving a broad range of ages and ethnic groups have found that both vaccines are highly effective and safe. These vaccines have been tested in tens of thousands of people.

Elizabeth Robilotti

Elizabeth Robilotti

Memorial Sloan Kettering infectious disease specialist Elizabeth Robilotti has answers to these frequently asked questions.

How are the vaccines administered?

Both the Pfizer-BioNTech vaccine and the Moderna vaccine are given as a shot in the upper arm, and both vaccines require two doses. The Pfizer-BioNTech vaccine requires two doses given 21 days apart. The Moderna vaccine requires two shots spaced 28 days apart. You must receive the same type of vaccine for your first and second shot.

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Is it still possible to get COVID-19, even after the second dose? How does the effectiveness compare with vaccines for other diseases?  

Both vaccines are among the most effective vaccines in history. They are as effective — if not more — than vaccines for polio, chicken pox, measles and the flu.   

The chances of getting sick after vaccination are minimal. Studies show even if you develop COVID-19 after being vaccinated, you are unlikely to get severely ill. Flu vaccines are less effective than the COVID vaccines, but they can protect you from more severe flu illness and hospitalization. The COVID-19 vaccines are even more powerful. Of the more than 30,000 people who received the vaccination during the research trials, only one developed a severe case. The efficacy of the vaccines in the prevention of severe COVID-19 is almost 100%.

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What are the side effects of each vaccine?

Both the Pfizer-BioNTech and Moderna vaccines reported similar side effects, which don’t last long — about one to three days. People receiving the vaccines reported pain at the injection site, fatigue, headache, and muscle pain. Side effects were more common after the second dose. Severe adverse reactions are rare.

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How do we know the vaccines will work well in all groups?

Both drug makers conducted rigorous studies that included people across a range of ages, gender, racial, and ethnic groups.

In the Pfizer-BioNTech study, 42% of people were over the age of 55, and 20% had other medical conditions. Minority ethnic groups represented 17% of vaccine volunteers.

In the Moderna study, about 25% of the people in the study were over age 65. The study group was 63% white, 20% Hispanic/Latinx, 10% Black/African-American, 4% Asian, 1% Native American, and less than 2% other.

Both vaccines offered strong protection against COVID-19 in all these groups.

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How effective are these vaccines?

Studies found they both work extremely well. The Pfizer-BioNTech vaccine was 95% effective in its study involving about 43,000 people. The Moderna vaccine was 94% effective in a study involving more than 30,000 people.

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Why are doctors confident these two vaccines are safe?

We have data from tens of thousands of people showing that the vaccines are safe based on an average of more than two months of follow-up. No major safety concerns have emerged. The CDC has robust tracking systems that will continue to monitor for any adverse events.

As for people with common allergies to medications, foods, inhalants, insects, and latex, they are no more likely than the general public to have an allergic reaction to the mRNA vaccines, according to guidance from the American College of Allergy, Asthma, and Immunology.

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How do the vaccines work?

Both are a new type of vaccine made from messenger RNA (mRNA). Unlike traditional vaccines, which put a weakened or inactivated germ into our bodies, mRNA vaccines teach cells how to make a protein that triggers an immune response if someone gets infected. When the vaccine is injected, the mRNA enters cells and tells them to start making the same protein that is found in the COVID-19 virus. The mRNA quickly breaks down in the body after triggering production of immune cells. Neither vaccine exposes you to the virus that causes COVID-19 or alters your DNA in any way.

What’s Different About Messenger RNA (mRNA) Vaccines for COVID-19?
Learn what is different about the messenger RNA vaccines that protect against COVID-19.
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Should people with cancer, who may have a weakened immune system, get the vaccines?

There should not be any increased risk for people with cancer to take either vaccine. The only possible difference in people with a severely weakened immune system is that the vaccines might not work as well in offering protection against COVID-19. But even in these patients, some protection is better than none.

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Are both vaccines safe for women who are pregnant or breastfeeding?

The CDC notes that pregnant women are at increased risk for severe illness from COVID-19 and recommends this group be given the opportunity to be vaccinated after discussing the risks and benefits with their healthcare provider. Neither of these vaccines were tested in pregnant or breastfeeding women, although animal studies have demonstrated that the vaccine is safe. The FDA has asked drug makers to study the effects on pregnant or breastfeeding women.

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Are the vaccines safe for me if I have already had COVID-19?

The CDC encourages people who have had COVID-19 to get vaccinated. Some of the people in the vaccine studies had evidence of a prior COVID-19 infection. They had side effects from the vaccine that were similar to those who had not been previously been infected.

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How long does the vaccine protection last? Will people need to get it annually, like the flu shot?

It is too early to tell how long the Pfizer-BioNTech and Moderna vaccines will provide protection against COVID-19, or whether follow-up shots will be needed on a regular basis. No vaccine provides 100% protection against getting the virus or spreading it to others, which is why it’s very important to continue following safety guidelines such as social distancing, wearing masks, and regularly washing hands. We will learn more as increasing numbers of people are vaccinated in the coming months.

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If I’ve received the COVID-19 vaccine, can I get other kinds of vaccines?

There should be two weeks between receiving the COVID-19 vaccine and any other vaccine, including the flu shot.

Information for Patients and Caregivers
Learn about our response to COVID-19 and our updated policies to protect our patients and staff.
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Will health insurance pay for the vaccines?

Based on what we know so far, the vaccine will be free. Vaccines are a critical part of public health efforts to prevent infectious diseases.


January 19, 2021


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