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April 16, 2021

How well Covid-19 vaccines work against variants, according to the latest research

Daily Briefing

    As more Americans receive Covid-19 vaccines, just how effective are vaccines at combatting variants of the new coronavirus that are surging throughout the United States? Here's what the latest research says.

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    The state of coronavirus variants

    According to The Hill, experts are scrambling to determine how well authorized vaccines perform against coronavirus variants because, in several countries, the variants are becoming the dominant version of the virus in circulation.

    For example, CDC Director Rochelle Walensky last week said the B.1.1.7 variant, which was first identified in the United Kingdom, is now the United States' dominant coronavirus strain, The Hill reports.

    The P.1 variant, which was first detected in Brazil, is now the second-most prevalent version of the virus in the United States, and the country also has reported increasing cases of the B.1.351 variant, which was first detected in South Africa.

    How effective are Covid-19 vaccines against variants?

    There is limited research available on how currently authorized vaccines perform against the variants. However, the research that is available—although generally preliminary—suggest that several vaccines are fairly effective at protecting against both the original version of the virus and the range of variants emerging around the globe.

    For instance, one study of Johnson & Johnson's vaccine—the distribution of which U.S. officials have temporarily paused—found it was 85% effective at preventing severe Covid-19 from the B.1.351 variant, which was first discovered in South Africa. Similarly, a small study of Pfizer/BioNTech's vaccine found it was up to 100% effective at preventing even mild cases from the B.1.351 variant.

    However, some preliminary research suggests not all vaccines are so effective against all variants. For example, one study of the vaccine developed by Novavax—which has not been authorized for use in the United States—found that, although it's about 89% effective at preventing mild Covid-19 from the original strain of the coronavirus, that efficacy drops to about 50% against B.1.351.

    And several lab experiments—in which blood samples from vaccinated people are exposed to variants or manufactured "pseudo-virus" variants—suggest that the antibodies produced by the Moderna vaccine are less effective against B.1.351 than against the original version of the virus. According to NPR's "Goats and Soda," these experiments suggest it takes about eight times as many of the antibodies produced by the Moderna vaccine to neutralize the B.1.351 variant as to neutralize the original version of the virus.

    That said, Salim Abdool Karim, an infectious disease researcher and co-chair of the Covid-19 advisory committee for South Africa, said he was not deeply concerned about those findings. "[T]he Moderna vaccine produces pretty high levels of antibodies," he said, "so there is enough antibody still to neutralize the virus."

    However, Abdool Karim expressed more concern about the efficacy of AstraZeneca's vaccine, which has not been authorized in the United States, against certain variants. One very small study suggested that that vaccine was almost entirely ineffective at preventing mild cases of Covid-19 against B.1.351, and a separate experiment found that it takes 86 times as many antibodies from the AstraZeneca vaccine to neutralize B.1.351 as it does to neutralize the original strain of the virus.

    "I'm basically not confident about [AstraZeneca's] vaccine at all" in mitigating B.1.351 infections, Abdool Karim said.

    What a real-world study revealed about the Pfizer/BioNTech vaccine's effectiveness against variants

    Amid these laboratory findings, researchers in Israel recently conducted the first real-world study—although still in pre-print—assessing the efficacy of the Pfizer/BioNTech vaccine against the B.1.1.7 and B.1.351 variants, with largely positive results.

    For the study, researchers compared almost 400 people in Israel who had been infected with the coronavirus after receiving at least one dose of the Pfizer/BioNTech vaccine with a control group of unvaccinated people who had similarly contracted the virus. Of those in the vaccinated group, 149 participants were infected at least one week after their second dose; the rest were infected more than two weeks following their first dose, but less than one week after their second dose.

    Overall, the researchers found that the vaccine performed well against all the variants circulating in Israel. However, because the vaccine doesn't have 100% efficacy, a few breakthrough infections occurred among vaccinated people—and those infections, while generally not severe, were most likely to be caused by the B.1.351 variant.

    Specifically, the researchers found that "vaccine effectiveness remains high" against the B.1.1.7 variant. "We see evidence for reduced vaccine effectiveness against the [B.1.1.7] variant, but after two doses, extremely high effectiveness kicks in," Adi Stern, a researcher at Tel Aviv University and senior author on the study, said.

    However, the researchers found that B.1.351 accounted for 5.4% of breakthrough infections among people who had received both doses and just 0.7% of the infections among unvaccinated people. "This means that the [B.1.351] variant is able, to some extent, to break through the vaccine's protection," Stern said.

    Even so, Stern noted that while the study wasn't able to pinpoint precisely how much lower the vaccine's efficacy was against the B.1.351 variant, she pointed out that "even if the [B.1.351] variant does break through the vaccine's protection, it has not spread widely through the population."

    Separately, Richard Lessells, an infectious disease expert focusing on the B.1.351 variant, said he doesn't believe the results of this study should "worry us unduly."

    The study results "seem to provide support to what we currently understand—that while the neutralizing antibody response is still developing post-vaccination and has not yet reached peak, there is still a risk of infection."

    "It is always important to keep in mind that vaccine protection is never 100%," Stern said. "As long as case counts are high, even fully vaccinated individuals should take precautions" (CIDRAP News, 4/12; Aizenman, "Goats and Soda," NPR, 4/9; Schumaker, ABC News, 4/12; Stein, "Shots," NPR, 4/15; Choi, The Hill, 4/11; Williams, The Hill, 4/09).

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