Do the COVID-19 Vaccines Protect Against Variants?

Microbiologist Esther Babady in her lab

Esther Babady is Director of MSK’s Clinical Microbiology Service.

New studies report encouraging results suggesting that the Pfizer-BioNTech and Moderna vaccines for COVID-19 are effective at protecting against severe disease caused by harmful variants of the virus.

The most important thing to know right now is that none of the current variants change the way people are tested, diagnosed, treated, or vaccinated.

Esther Babady, Director of MSK’s Clinical Microbiology Service, is a nationally recognized leader in testing for SARS-CoV-2, the virus that causes COVID-19. Here, she answers questions you may have.

If I’m exposed to any of the current variant strains of COVID-19 after I’m vaccinated, will I be protected?

Yes. According to the Centers for Disease Control and Prevention (CDC), the COVID-19 vaccines offer protection against infection by most variants. Most important, they have prevented serious illness, hospitalization, and death, even at a time when new variants are spreading around the world.

Two new studies based on real-world use of the vaccine, one conducted in Israel and one in Qatar, suggested that the Pfizer vaccine prevents severe outcomes, including pneumonia and death, in cases of COVID-19 caused by the variants B.1.1.7 (a strain that first appeared in the UK) and B.1.351 (a strain that first appeared in South Africa).

A preliminary study from Moderna reported that a booster shot it developed protected people from the variants B.1.351 (South Africa) and P.1 (a strain that first appeared in Brazil).

Early results from Johnson & Johnson suggest that its vaccine is also effective against severe disease caused by the variants that are currently circulating.

COVID-19 Vaccine Information for Patients
Learn what you need to know about the COVID-19 vaccines if you are an MSK patient.
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How do we know which variants are important?

When scientists find variants, it means the virus is changing as it moves through the population. It’s completely normal and expected. The CDC classifies variants into three categories:

  • Variants of interest. They may impact transmission, diagnosis, or response to therapy. They include two strains that emerged from New York (B.1526 and B.1525) and one from Brazil (P.2). Most recently, the CDC added four strains that have emerged from India (B.1.617, B.1.617.1, B.1.617.2, and B.1.617.3).
  • Variants of concern. In addition to the impact mentioned above, these strains may also lead to more severe illness. They include the strain that emerged from the UK (B.1.1.7), the one from South Africa (B.1.351), one of the strains from Brazil (P.1), and two from California (B.1.429 and B.1.427).
  • Variants of high consequence. These strains may not be detected by current tests and may not respond to current therapies or be prevented by current vaccines.

Thankfully, no variants in the third category have been found at this time.

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Have MSK patients or staff been infected with a COVID-19 variant after being vaccinated?

Yes, but very few. MSK has vaccinated tens of thousands of employees and patients, and we have identified fewer than 100 people who have experienced a breakthrough infection after the second dose. No vaccine is 100% effective, so this small number of breakthrough infections was expected.

About half of the breakthrough cases were found to have variants, and among those we found a distribution of the different variants. This is good news because it means that at this time, variants are not driving breakthrough infections in vaccinated people. It also means there is not one single variant that’s leading to a greater number of infections after vaccination.

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Will I need to get a booster shot?

In addition to Moderna, other vaccine manufacturers are already working on developing booster shots that will target the variants, but it’s not clear how soon they may be needed.

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When did MSK begin testing for COVID-19 variants?

We have been regularly testing for variants since January 2021 and have processed about 1,000 samples. We look for variants in every patient and staff member who tests positive for COVID-19 unless there’s not enough virus to test.

Testing enables us to detect the frequency of known variants and also determine whether newer, emerging variants are present.

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Which are the most common variants among MSK patients and staff?

B.1.1.7 (the one that emerged from the UK) is by far the most common one that we’ve seen, with 70 cases as of April 22, 2021. But we’ve seen at least one case of each of the other strains mentioned above as well, with the exception of the new ones that have emerged from India.

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How do MSK’s findings about variants compare to those reported in New York City?

In New York City as well as at MSK, the B.1.1.7 (UK) variant is the most common strain that’s been detected. Overall, new strains seem to appear in New York a week or two before they appear in MSK’s population. We’re not sure why that’s the case. It does take about a week for us to get results for many of our samples, so that could explain part of the delay.

The bottom line is that the faster people are vaccinated, the less chance the virus will have to develop additional variants.


May 7, 2021


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