When we talk about the Covid-19 vaccines, we aren’t necessarily talking about people getting one shot, then being done and good to go. Both the Pfizer and Moderna vaccines — which are the closest to approval — have two-dose regimens. You get one shot, and then a few weeks later, you’re supposed to get another one.
Among all the logistical challenges in distributing and administering a Covid-19 vaccine, this looks like one of the biggest.
That’s because, based on research that evaluated compliance with other multi-dose vaccines, patients are really, really bad at getting their second dose. Bad as in, as many as half of patients never do. Studies conducted in both the US and UK on the hepatitis B vaccine — which, like the Covid-19 vaccines, is supposed to have around a one-month period between the first and second doses — found that roughly 50 percent of patients failed to get their follow-up shot within a year after their first.
Given the urgency of ending the pandemic, we can hope for better compliance with the Covid-19 shots. But those findings are, as Ateev Mehrotra at Harvard Medical School put it to me, “humbling.”
“While I recognize the situation now is different and the rates of completion will almost certainly be much higher, these prior studies highlight that the logistical barriers with a two-dose vaccine are enormous,” he said over email.
The results from the UK study, written by researchers for the pharma giant Merck, “are worse than even I had expected,” Mehrotra said, with 46 percent of patients receiving the second dose within the recommended period (one month after the first dose). Even when the follow-up period was extended out to 13 months, just 54 percent of patients had been vaccinated with both doses as recommended by the manufacturer.
Adherence was dismal in the US study as well, which was also conducted with Merck’s support.
If a large number of the people who begin their vaccination course fail to get full protection because they skip a second dose, then it’s only going to be more difficult to stop the coronavirus’s spread.
That doesn’t mean the Covid-19 vaccination campaign is doomed, though. For one, new clinical trial results show these two-dose vaccines might still be somewhat effective if the patient gets only one dose. But full compliance would of course be the ideal, and the US government is already working on plans to try to encourage better adherence to the two-shot schedule.
We need people to get their second dose — or hope a single dose is still effective
There is another complication in getting people their second dose: side effects. The hepatitis B vaccines do not really have meaningful side effects. But these new Covid-19 vaccines do.
“These new vaccines do result in a lot of flu-like symptoms. That is a sign that the vaccine is working, but I do fear that will be a barrier for people getting the second shot,” Mehrotra told me. “It is harder to get a shot when you know it may knock you out for a day.”
“You’ll see a bunch of ideas on how to improve compliance,” Mehrotra told me. “But I don’t think we have any solutions that increase this to the levels we might want.”
That raises the question: What happens if somebody gets only one dose of a two-dose Covid-19 vaccine?
The Food and Drug Administration reported some tentatively good news Tuesday on that issue, releasing results that show one dose of the Pfizer vaccine still provides some protection against Covid-19 and that having only one dose is safe for the patient. The Pfizer vaccine was found to be more than 50 percent effective after the first dose.