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Why no one should skip the second dose of the COVID-19 vaccine

(Marijan Murat/dpa via AP)

While Washington state fights slowing COVID-19 vaccine demand and tries to fill the gap between those fully vaccinated and the threshold for herd immunity, people who do not go back for their second dose are creating another challenge.

According to statistics released by the Washington State Department of Health, about 13% of Washingtonians who get the two-dose Pfizer or Moderna — which require three weeks and four weeks in between doses, respectively — are behind on getting their second dose. Just under 5% never got it at all.

State Secretary of Health Dr. Umair Shah said there could be a variety of reasons for the delay.

“There’s confusion that they need a second dose, there’s some confusion that Johnson & Johnson is one dose, the other two are two,” he said. “Sometimes people are just misplacing the appointment times, dates, and then thinking mistakenly, ‘Well, if I’ve already missed it, I can’t go back.'”

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Others could be looking to avoid the side effects that can come with the second shot, figuring that the partial protection from the first is good enough.

However, Dr. Edward Leonard, an infectious disease expert at Overlake Medical Center, said this kind of thinking is a mistake. While we do get the majority of our immunity from the first dose — around 60% for Pfizer, and 80% percent for Moderna — it is still incredibly important to go back for the second dose.

Leonard explained that the two doses function in different ways. The first dose comes in and creates antibodies over a period of a few weeks. But after those antibodies are created and the body has learned about how to fight the disease, the second dose is what really kicks the immune system into gear.

“The first dose kind of gets things going, and then the second dose comes behind and boosts it even more, so that the immune response creates even more antibodies,” he said.

He compared the process of the body learning about the coronavirus with the learning that takes place before school exams.

“If you think about studying in school, and you take a midterm, think of that as the first dose — you’re getting the first amount of learning of the material,” he said. “And then the final exam is the booster dose. So you’ve studied more, you’ve got more information.”

Getting the second dose of Pfizer or Moderna not only boosts your immunity, but it also keeps you immune longer — scientists believe that immunity could last up to a year. The current thinking is that like the annual flu shot, people will be protected for a year at a time.

“It’s critical because it provides the appropriate booster response to get to that 94, 95% efficacy,” Leonard said, adding, “And then if you did only one shot, it may not last that full year.”

Additionally, there are impacts on herd immunity in skipping the second dose. If people have just one dose, they cannot be counted in the 80% or so of fully-vaccinated people needed for herd immunity.

“In the setting of a pandemic as we’re dealing with now, the more antibodies we can get people to have, the better we can get to that elusive herd immunity,” Leonard said.

Dr. Leonard is hopeful the state can get to herd immunity at some point, but he predicts it will be a long road.

If every person who gets their first dose gets the second dose, that will help — so even if you’re late, go and get that second shot. Health experts say getting the shot a bit late will still be far more effective than not getting it at all.

And while showing up early is usually preferred in society to being late, Leonard cautions people not to get the second shot early. Getting Pfizer too far before the three weeks or Moderna before the four weeks can impede the antibody response.

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