COVID-19 Vaccine Safety, Effectiveness, and Other Important Information

An MSK employee after receiving a COVID-19 vaccine

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

As we learn more about COVID-19 variants, we know that vaccination remains our most effective weapon against this tenacious virus and our best opportunity to end this pandemic.

Mini Kamboj, MSK’s Chief Epidemiologist, answers some frequently asked questions about the COVID-19 vaccines and how they work.

How effective are the COVID-19 vaccines?

The COVID-19 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 vaccines all provide strong protection against severe disease, hospitalization, and death from COVID-19.

  • On August 23, 2021, the Pfizer-BioNTech COVID-19 vaccine (Comirnaty®) received full approval from the Food and Drug Administration (FDA). The federal agency issued its decision after a thorough review of Pfizer-BioNTech’s updated clinical trial and manufacturing data, which showed that the vaccine is 91% effective in preventing infection, hospitalization, and death from COVID-19. It was previously administered under an Emergency Use Authorization.
  • On January 31, 2022, the Moderna COVID-19 vaccine (Spikevax™) received full approval from the FDA. The federal agency issued its decision after a thorough review of clinical trial and manufacturing data, which showed that the vaccine is 93% effective in preventing COVID-19 and 98% effective in preventing severe disease.
  • The Johnson & Johnson/Janssen (J & J) vaccine has been shown to be 66% effective.
  • In July 2022, the Novavax COVID-19 vaccine received Emergency Use Authorization from the FDA. This vaccine was studied in a clinical trial of about 40,000 people. It was shown to be 90% effective at preventing mild, moderate, and severe COVID-19 infection. However, this trial was conducted before the emergence of the Omicron variant.
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How do the COVID-19 vaccines work?

The vaccines all work by teaching cells how to make one part of the COVID-19 virus (the spike protein) in order to trigger an immune response. What those instructions look like and how they make their way inside your body is a little bit different depending on which vaccine you get.

In all cases, the vaccine trains your body to recognize the virus so that if you are infected, your immune system is ready to fight it.

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Do the COVID-19 vaccines protect against variants?

Research is ongoing. Because the vaccines are extremely effective overall, it has been seen that they are also at least somewhat effective against emerging variants.

Read more about effectiveness of vaccines against variants »

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How long does the vaccine protection last?

No vaccine protects forever against getting the virus or spreading it to others. That’s why it’s very important to continue following safety guidelines, such as social distancing, wearing a mask in some situations, and regularly washing hands.

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Are MSK employees required to be vaccinated?

Yes. All MSK employees must be vaccinated against COVID-19 as a condition of their employment. MSK is following the New York State and New Jersey COVID-19 vaccine mandates for healthcare workers.

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The COVID-19 vaccines were developed so quickly. How can I be sure that the COVID-19 vaccines are safe?

The COVID-19 vaccines were developed fast because scientists had a head start. The technology already existed and using it to develop a vaccine for a new virus became an international priority, unlocking billions of dollars to ensure safety while moving urgently to save lives. Regulators streamlined some steps in the authorization process, but the vaccines still had to meet the FDA’s rigorous safety and effectiveness standards. The CDC continues to carefully track all adverse reactions.

A few extremely rare instances of severe side effects have emerged and been investigated, but scientists have concluded the risk of getting severely ill from COVID-19 is far greater than the risk of these side effects. Here is proof of this careful and ongoing scrutiny:

  • In mid-July 2021, the CDC said that about 100 out of 13 million Americans who received the J & J vaccination developed Guillain-Barré syndrome. Guillain-Barré syndrome is a rare neurological condition in which the body’s immune system attacks nerve cells. Read more »
  • In late June 2021, the CDC reported cases of heart inflammation potentially linked to the Pfizer-BioNTech and Moderna vaccines. The heart issues were very rare. In the clinical trial testing the Novavax COVID-19 vaccine, there were also some cases of heart inflammation. Currently, it is not possible to compare the risk of heart inflammation between the different vaccines. Read more »
  • As of late November 2021, the CDC and FDA had identified 54 people who were diagnosed with a blood clotting disorder called thrombocytopenia syndrome (TTS) after receiving the J & J vaccine (among 16.4 million doses). On December 16, the CDC recommended that Americans receive either the Pfizer-BioNTech or Moderna vaccine rather than the J & J vaccine. Read more »

There is incorrect information from unreliable sources on social media about how the Pfizer-BioNTech and Moderna vaccines use something called messenger RNA (mRNA). The facts are that there are thousands of different kinds of mRNA in human cells. Each kind of mRNA does different things.

It’s important to know mRNA is not the same as DNA and cannot be combined with DNA to change your genetic code.

Here’s how the mRNA vaccines work: They use a tiny piece of the coronavirus’ genetic code to teach your immune system how to make a protein that will trigger an immune response if you get infected. The mRNA is fragile and it delivers the instructions to your cells to make antibodies against SARS-CoV-2. The mRNA does not enter the nucleus of the cell — the part that contains your DNA.

The vaccines do not cause cancer. They do not expose you to the virus that causes COVID-19. You cannot get COVID-19 or any other infection from the vaccine. Read more about how mRNA vaccines work »

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Should people with cancer get the COVID-19 vaccines?

Yes. We strongly recommend that our patients with cancer get the COVID-19 vaccine. Some cancer treatments like chemotherapy or radiation can reduce how well a person’s immune system responds. It’s possible that the COVID-19 vaccines may not be as effective in those patients. Even so, some protection is better than none.

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Are COVID-19 vaccines safe for women who are trying to get pregnant, are pregnant, or breastfeeding?

Yes, the vaccines are safe and are recommended for people who are trying to get pregnant, are pregnant, or are breastfeeding. In late September 2021, the CDC issued a strong recommendation for people who are pregnant or are trying to become pregnant to get vaccinated. This is because pregnant people are at higher risk for severe COVID-19 illness and adverse pregnancy outcomes, including preterm birth.

There is no evidence that COVID-19 vaccines cause any problems with pregnancy, including development of the placenta. As of November 29, 2021, more than 177,000 people registered with the CDC have indicated that they were pregnant when they were vaccinated. A group of them are being closely monitored to see how the vaccine affects pregnant people. No safety concerns have emerged.

Research has shown that pregnant people who are vaccinated not only protect themselves but also their babies, by passing on antibodies in their umbilical cord and breast milk.

There is no evidence that the COVID-19 vaccines cause infertility in women or men.

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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 infected.

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What are the side effects of each vaccine?

The vaccines reported similar side effects, which don’t last long — about 1 to 3 days. People receiving the vaccines reported pain at the injection site, fatigue, headache, and muscle pain. Severe adverse reactions are rare.

People with common allergies to medications, foods, inhalants, insects, and latex 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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Can I do anything ahead of time to reduce any side effects from the vaccine?

You should wait until after being vaccinated to see how you feel. If you do experience side effects, it is OK to take an over-the-counter drug like Advil or Tylenol to lower a fever, reduce chills, or relieve a headache or body aches.

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Do the COVID-19 vaccines contain animal products?

No, the vaccines do not contain animal products or animal-derived ingredients such as pork, shark squalene, gelatin, cholesterol, egg, or milk. In addition, none of them contain preservatives, gelatin or latex. All COVID-19 vaccines also are free from metals such as iron, nickel, cobalt, lithium, rare earth alloys or any manufactured products such as microelectronics, electrodes, carbon nanotubes, or nanowire semiconductors.

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Should I get an antibody test after the COVID-19 vaccine to make sure it’s working?

Antibody testing is not currently recommended to check someone’s immunity to COVID-19 following vaccination. The COVID-19 antibody test used at MSK detects the immune response after being infected by COVID-19; it does not measure the immune response as a result of the vaccine. That’s why it should not be routinely ordered to assess vaccine response.


August 8, 2022


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