"It's certainly concerning," said Dr. Thomas Holland, a professor of infectious diseases at Duke University. "It's really important to get that second dose."
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In a report just released, CDC data shows that through April 9, 92% of people who received one dose of an mRNA vaccine also got their second dose within the scheduled window.
About 8%, or 5 million people, however, may have missed that second dose, according the CDC.
Dr. Holland said many people may be feeling a false sense of security after the first dose, and it's the second dose of the mRNA vaccines that provide long-lasting, durable protection against the virus.
For example, based on a recent real world U.S. study, the CDC said the first dose of the Pfizer and Moderna vaccine was 80% effective at protecting against COVID-19 infection two weeks after participants got their shot; that level of effectiveness rose to 90% after their second dose.
"The first dose of the vaccine acts as a primer and the second dose is the boost that really gives higher antibody levels," he said. "People experience this when they tend to get more side effects with the second dose because their immune system is ready and it's primed and that's evidence of the immune system is working and it's having the desired response."
Holland said it's those dreaded side effects that may be keeping people from making that second dose appointment, but he added -- along with reports of second doses not being available on time -- people have also just been busy, forgotten their appointment, or didn't have a way to get there.
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While Pfizer and Moderna have recommended windows for a second dose of 21 and 28 days respectively, the CDC said patients can wait up to six weeks to get that second and final shot.
Holland said no matter how long it's been since you're first dose, it's best to get that second dose as soon as possible.
"If you're a little bit delayed getting that second dose, it's not too late," he said. "Go back and get it. You will still get good immune protection if that dose is not right at the 21-day mark. Find a place where the vaccine is available and go ahead and get that second dose as soon as you can."
For those who have already had COVID, Holland said there are reports that the first dose of an mRNA vaccine could act as the booster, but he urged two doses is still the best approach to fighting the virus and reaching herd immunity.