People who get one of the new COVID-19 vaccines will be expected to get their second shot 21 or 28 days after the first one, depending on the manufacturer. But what happens if someone misses that deadline by a day, a week or even longer?
"There is the regulatory answer and there is the scientific observation," Moncef Slaoui, co-leader of Operation Warp Speed, the federal government's vaccine development effort, said in a Wednesday news conference.
Regulators are likely to authorize both vaccines – one likely within the next four days and the other a week later – with the expectation that people receive them on the schedule by which they were tested in clinical trials. The vaccine made by Pfizer/BioNTech is to be given in two doses 21 days apart, while Moderna's vaccine was studied with the two doses coming 28 days apart.
From a scientific perspective, such precision is not that important and the immune system generally responds better when there's a wider gap between vaccinations, said Slaoui, an immunologist who spent a 30-year career in vaccine development.
But during a pandemic, when the risk of infection is high, he noted that people are better off getting the second shot – and being fully protected – according to the authorized schedule.
"If there is significant transmission of disease, as is the case here, we should absolutely get the second dose exactly as has been studied," he said.
Data released Tuesday by the U.S. Food and Drug Administration, shows that protection from the Pfizer/BioNTech vaccine starts almost immediately after the first shot.
"Hopefully, we will start affecting people's lives very quickly after the onset of campaigns to immunize," Slaoui said.
But the level of protection increases dramatically – from 52% to 95% – after the second shot, he said, and traditionally second shots provide longer-lasting immunity than a single dose. Everyone in the study got a second shot roughly 21 days after the first, so it's unclear whether protection from a single dose would be long-lasting.
Slaoui strongly discouraged people from getting just one dose, because it wasn't studied that way.
"I'm absolutely not suggesting that the vaccine should be a one-dose vaccine," he said, though perhaps future studies could look at a one-dose regimen.
A third candidate vaccine being developed by Johnson & Johnson may require only one shot. The company is currently studying JNJ-78436735 as both a two-dose and a one-dose regimen.
Slaoui said 38,000 people out of 40,000 already have been enrolled in the J&J trials. The company will likely release safety and effectiveness data early next year, he said, at which point it will become clear whether the single dose is adequately protective.
Assuming one of the two approaches proves safe and effective, the FDA is likely to authorize use of that vaccine in late January or early February, he said.
Slaoui said he hopes all of the vaccines will provide long-lasting protection, and the immune data he's seen so far is encouraging. Trial participants are only required to be followed for two months to receive emergency authorization from the FDA, but companies plan to continue to follow them for 2 years to ensure longer lasting safety and effectiveness.
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This article originally appeared on USA TODAY: COVID vaccine: Timing of second Pfizer and Moderna shots not critical