| In this week's edition of the Covid Q&A, we look at immunity after vaccination. 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 John. He wonders whether it's safe to plan travel or other activities that would take place months after vaccination — would he still be immune to the virus?
How will one know how long a Covid vaccine is effective? As more people get vaccinated, we're starting to imagine what life might look like when things go back to normal. Things like going to the movies or having a birthday party — even international travel — suddenly seem within reach again.
The U.S. Centers for Disease Control and Prevention advises that those who are fully vaccinated can hang out with each other indoors without social distancing. But it's still important to maintain precautions in public and around unvaccinated people for a few reasons. Though vaccines do seem to at least greatly reduce transmission, it's still unclear whether vaccinated people can still spread the virus. And as we discussed here a few weeks ago, vaccines appear to be effective for the vast majority of people but not everyone. That means if you're vaccinated, there's still a slim chance you could get ill if exposed to the virus. Just this week, there have been a handful of such cases reported in the U.S. For those reasons, the CDC is still advising against unnecessary travel. At least for now, even vaccinated people still carry the risk of catching the virus in crowds.  A man gets the Pfizer vaccine at a pop-up vaccination clinic in New York City. Photographer: Spencer Platt/Getty Images Of course, as more people get vaccinated, the assumption is that transmission will drastically slow, if not stop. So let's get to John's question about how long immunity conferred by vaccines lasts.
Chunhuei Chi, director of the Center for Global Health at Oregon State University, says it's a complicated question without much data available to answer it. The urgency of creating a vaccine during a pandemic meant that drug companies didn't have time to look at that question, he says.
"Reports from earlier clinical trials suggested that such immunity will last at least six months," Chi says. "Many experts have a more optimistic estimation of the duration of immunity, based on research findings from people who recovered from Covid-19 infection. They found those infection-induced immunities can last eight months or even longer, especially for people who were infected and developed severe symptoms." On Friday, Pfizer and partner BioNTech released data that support Chi's view. They said their vaccine was 91.3% effective in preventing symptomatic cases starting one week after the second dose through as long as six months.
Lana Dbeibo, an infectious disease expert at Indiana University School of Medicine, said it is possible that we might need booster vaccines as we do for flu shots.
"But whether this will indeed be the case for Covid, and if so, what the length of time is between the initial vaccination and the booster doses, is not yet known," she says.
And for the time being, it's not possible to simply test yourself to see if you're vaccine is working.
This lack of certainty is frustrating, for sure, but more answers are coming every day. There's nothing wrong with a little optimism. But if you do book a vacation for a few months from now, just maybe make sure it's refundable.
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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