| In this week's edition of the Covid Q&A, we look at how to know whether a vaccine is working. In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis is picking one question sent in by readers and putting it to an expert in the field. This week's question comes to us from Bill in Westland, Michigan. Bill wants to know if there's a way to know whether his vaccine is working. Bill asks:
Is there a way for an individual to test their Covid-19 vaccine's efficacy after the second shot?
What a great question! More than 133 million doses of Covid vaccines have been administered in the U.S. We know from large clinical trials that those vaccines work for the vast majority of people, but not everyone. The double-dose Pfizer and Moderna vaccines were shown to have efficacy rates of about 95%, while the single-shot Johnson & Johnson vaccine had a rate of 72%. (Click here to read a previous Covid Q&A on vaccine efficacy.)
So how do you know if yours is working?
It would be nice if there were a test you could take after vaccination that told you conclusively whether you were protected from Covid-19. But unfortunately it's not that simple, says Robert Murphy, an infectious- disease expert at Northwestern University.
"There is no way to reliably do this with the available commercial tests," he says. It seems logical that you would be able to take an antibody test to figure out whether you had developed immunity to Covid, but the tests weren't created to do that, he says. "The commercial tests are not good enough, nor were they designed to evaluate vaccine response," Murphy says. "They were designed to determine if there was prior infection. The formal recommendation is not bother to do any testing after receiving the vaccine doses and to assume one is optimally immunized two weeks after the last dose."  Home antibody tests were designed to test for prior infection, not vaccine efficacy, says Robert Murphy, an infectious- disease expert at Northwestern University. Photographer: Gem Atkinson/Bloomberg In other words, we know the vaccines are effective for an overwhelming majority of people. And those odds are a better gauge of whether a vaccine might work for an individual than any commercially available antibody test. There are experimental ways of figuring out immunity post-vaccination, but they're not yet available to the general public. "When you have an efficacy rate of 95%, it's hard to imagine a commercially available antibody test will provide you with any useful information," Murphy says. That is one reason why it's important to keep up social distancing and mask-wearing, even after vaccination. And it's why it's so important for everyone to get inoculated. If enough people get vaccinated, then there just simply won't be enough virus in circulation to affect the small number of people for whom the vaccines might not work.
Thanks to all of you for writing in this week! Next Sunday, we'll be answering the best question we receive again. So if you have any, we want to hear from you. Write to us at CovidQs@bloomberg.net — Kristen V. Brown | 
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