WASHINGTON — Question:
What happens if I skip the second dose of the COVID-19 vaccine? What if I get the second dose early or late?
According to an FDA analysis of the Pfizer vaccine study, there does appear to be "some protection against COVID-19," after just one dose of the vaccine. However, there is limited data on this subject, and the effectiveness increases greatly after a second dose. After two doses, there is an efficacy rate of 95 percent.
Infectious disease specialists told the VERIFY Team that missing the suggested dates by a couple days is probably not a major issue. However our experts said that getting the vaccine significantly too early or late can impact the effectiveness of the vaccine.
- FDA Analysis of the Pfizer Vaccine, December 10, 2020
- Dr. Amesh Adalja, Senior Scholar at the Johns Hopkins University Center for Health Security; Infectious Disease Specialist
- Dr. Linda Nabha, University of Pittsburgh Department of Medicine, Infectious Disease Specialist
On social media, there are many questions about the Pfizer vaccine, which requires two doses, separated by 21 days. The Moderna vaccine would also need two doses, separated by 28 days.
The Verify team tackled two questions, relating to the vaccines' second dose; What happens if you skip the second dose, and what happens if you take the second dose earlier or later than suggested.
Dr. Amesh Adalja, a Senior Scholar at the Johns Hopkins University Center for Health Security, said that preliminary data indicates that there is likely some protection from the virus from the first dose.
"We know that there is some immunity that people will get after a second dose," he said. "But it may not be as durable, it might not last as long, and it may not be as strong as when you get two doses."
An FDA analysis of the Pfizer vaccine study indicates this point.
After two doses, the vaccine had an efficacy rate of 95 percent. Meanwhile, there did appear to be some protection from the virus after just one dose.
"Based on the number of cases accumulated after Dose one," the analysis read. "And before Dose two, there does seem to be some protection against COVID-19 disease following one dose."
The preliminary data from the study indicated that the efficacy rate after just one dose was 52.4 percent. However, the analysis pointed out that it's impossible to generate any conclusions from this number.
"The efficacy observed after Dose one and before Dose two," the analysis read. "From a post-hoc analysis, cannot support a conclusion on the efficacy of a single dose of the vaccine, because the time of observation is limited by the fact that most of the participants received a second dose after three weeks. The trial did not have a single-dose arm to make an adequate comparison."
Dr. Linda Nabha, an infectious disease specialist at the University of Pittsburgh Department of Medicine, said the second dose will boost the effectiveness of this vaccine.
"That first dose is going to be that sort of prime vaccine," she said. "It primes up that immune system. And what that second vaccine does is it boosts that prime immune system. And it gives you that optimal antibody response that we were really looking for."
The Verify team asked our infectious disease specialists about the consequences of taking the second dose earlier or later than suggested. The second doses of the Pfizer and Moderna vaccines are meant to be taken 21 and 28 days later, respectively.
Our experts agreed that the bigger concern would be if the second dose were taken too early, which would limit the effectiveness.
"If you get the second dose just a day or so too early," said Dr. Adalja. "It probably doesn't make too much of a difference. If it is very early, you are likely to not get the benefit of the second dose. And it's basically de facto - you have a single dose vaccine, and you won't have the benefit of someone who had two doses spaced appropriately."
Dr. Nabha said that there is more flexibility for those who get the second vaccine later than requested.
"If you're missing it by a few weeks," she said. "It's highly unlikely to be a significant issue. But if we're talking about several months, you might need to be re-vaccinated."
Overall, our experts agree that the best course of action is to get the second dose when requested.
"You want to stick to that second date as closely as possible," said Dr. Nabha.