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October 28, 2020

Why a 'mediocre' coronavirus vaccine could still be a game-changer

Daily Briefing

    Under current FDA guidelines, America's first coronavirus vaccine could be just 50% effective, making it "mediocre" when it comes to efficacy, F. Perry Wilson, an associate professor of medicine at the Yale School of Medicine and director of Yale's Clinical and Translational Research Accelerator, writes for Vox. But that doesn't mean you shouldn't get it, Wilson notes. Here's why.

    A vaccine approval is coming. Get ready to ask these 8 questions.

    The 'mediocre vaccine paradox'

    Wilson writes that, "[t]hough nothing is certain, it's very likely that" federal regulators will authorize "some vaccine" against the novel coronavirus "soon"—and that the vaccine will be "widely available by the end of 2021."

    However, Wilson writes that it's possible America's first coronavirus vaccine won't be "very good," having just a 50% efficacy. That's because FDA "[C]ommissioner Stephen Hahn has pledged that the FDA will not grant approval for a vaccine that has less than 50% efficacy, so this seems like a reasonable floor," Wilson writes.

    To put that level of efficacy into perspective, Wilson notes that major drugmakers currently conducting late-stage clinical trials on coronavirus vaccine candidates each are enrolling "30,000 to 60,000" participants who haven't yet been infected with the virus. Through those trials, half of the participants will be given the experimental vaccines, while the other half will receive a placebo.

    According to Wilson, when evaluating the outcomes of those trials, "[t]he best-case scenario" is that "[z]ero cases in the vaccinated group and something well north of zero cases in the placebo group" become infected with the novel coronavirus. In such a scenario, "[w]ith, say, 150 cases in the placebo group and zero in the vaccine group, [the drugmakers] would be able to claim that their vaccine was nearly 100% effective," Wilson explains. In comparison, he writes, "[i]f there are 150 cases in the placebo group and 75 cases in the vaccine group, now they could say that their vaccine is 50% effective"—results that aren't "as impressive, clearly, but" that still show "something is … happening."

    However, having a "mediocre vaccine" when it comes to efficacy presents a "problem," Wilson writes, because "[t]he less effective a vaccine is, the more of us need to get it to end the pandemic."

    For example, he writes, "[l]et's say that, on average, an individual infected with" the novel coronavirus "infects two more people (the now familiar R0)." In that case, to stop the virus's spread, "we need to make sure that each infected person infects less than one additional person."

    According to Wilson, to achieve such an RO without mitigation measures such as mask wearing, social distancing, avoiding large gatherings, and others, "one out of every two people" must be immune to the novel coronavirus. So, he explains, "[i]f there is a 100% effective vaccine, and one out of every two people need to be immune, it means that 50% of the population would need to be vaccinated to end the pandemic."

    But in the scenario of a 50% effective vaccine, the proportion of the population that would need to get vaccinated doubles. "In that case, one out of every two people who is vaccinated won't be protected," Wilson explains. So "[t]o end the pandemic under this scenario, 100% of the population would need to be vaccinated," he writes.

    Why people should get the vaccine—even if it's not very effective

    Recent research shows that the top predictor of whether people are likely to get a coronavirus vaccine is the vaccine's effectiveness, with people, on average, saying they are 16% more likely to get a vaccine if it's 90% effective compared with one that's 50% effective, Wilson notes.

    However, Wilson argues that getting even a "mediocre vaccine" still could help to end the coronavirus pandemic when paired with other mitigation efforts.

    "The key is the R0," Wilson writes. "My example assumed that the average infected person infects two more people, but it doesn't have to be that way. In fact, due to the policies enacted to stem the tide of the virus, the effective R-value (often called Rt) is probably lower than two—maybe around 1.2."

    In such a case, Wilson writes, "you'd only need to get a 50% effective vaccine to 33% of the population to end the pandemic, provided we all continue to wear masks until it's over" (though he also notes that those estimates could change, depending on whether a vaccine is more or less effective in certain populations.)

    Side effects will be key to consider, too

    Although efficacy is a key factor to consider when deciding whether to get a vaccine against the novel coronavirus, it's not the only factor, Wilson writes. "The side effects are critical, too."

    Wilson argues that people shouldn't "be too worried about non-serious adverse events" associated with coronavirus vaccine candidates, which can include things like "[r]edness and swelling at the infection site." He notes, "Every vaccine will have them: fevers, injection site reactions, malaise."

    But people do "need to worry about" any "serious adverse events," he writes. And while "fortunately these are usually rare," there's no way to know the true rate at which they can occur, Wilson explains.

    As such, Wilson writes, "[w]e need some way to estimate a plausible side-effect risk, given the observed data," and "[t]his can be done with some simple statistics." Based on those statistics, Wilson explains, "For serious adverse events that don't occur at all in the [late-stage trials], we can be confident that the true rate is less than one in 10,000. That is a really small number, but not necessarily reassuring when we're talking about vaccinating 350 million Americans (or nearly eight billion people)."

    Side-effect risks may not mean you should wait to get vaccinated

    In light of those statistics, some people may be inclined to think it's best to wait until a large number of other people are vaccinated to see the true rate of serious side effects that could emerge, Wilson writes. But that presents "a bit of moral hazard," Wilson explains, because "if we all act this way, we'll never learn anything and the pandemic will march on."

    Overall, whether a vaccine is able to help stop the coronavirus' spread in America will largely rely on "a particular weakness in American society" that the coronavirus epidemic has highlighted: "a seemingly irreconcilable tension between a perception of individual liberty and collective sacrifice," Wilson writes.

    According to Wilson, "Mediocre vaccines are more like face masks than miracles," as "[t]he real benefit of getting vaccinated may be less to the individual and more to society." And because of that possibility, he writes, "[w]e need to prepare the public for a less-than-ideal scenario—a mediocre vaccine—and show why it is important to get vaccinated anyway. A mediocre vaccine won't necessarily save you, but if enough of us get it, it can save us" (Wilson, Vox, 10/22).

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