Here's Why One Coronavirus Vaccine Shot Won't Be Enough

Nearly all vaccine candidates being tested are expected to require two doses, experts say.

With new information continuing to come out regarding the progress towards developing and distributing an effective coronavirus vaccine, medical professionals are learning more and more about what the end result might look like. Most recently, several such experts speaking to USA Today revealed that if and when a successful vaccine is developed it will likely require two shots, and quite possibly a booster down the road.

"As far as I am aware, with one set of exceptions, all the front-line vaccine developers are contemplating two shots," Barry Bloom, PhD, an immunologist and professor of public health at the Harvard T.H. Chan School of Public Health, told the newspaper. "The one exception is Merck, which last week pushed forward on two vaccines, each of which they hoped would be one-shot vaccines."

While it's too early to say for sure at this point, the medical community will have a much better idea of whether or not a two-shot regimen will be necessary based on the results of humane trials—which in some cases are expected to commence in July, according to a recent interview that Anthony Fauci, MD, did with the Journal of the American Medical Association.

The reason why administrating two doses could be necessary will be based on how the immune system reacts to the initial dose, L.J. Tan, chief strategy officer with the Immunization Action Coalition and co-chair of the National Adult Immunization Summit and National Influenza Vaccine Summit, explained to USA Today.

"The immune system looks at it and it processes and remembers it, developing antibodies and immune cells," Tan said. "If an infection comes at that point you'll fight it off and you won't get sick—you'll be immune."

However, with some viruses—particularly ones that your body has never been previously exposed to, like the coronavirus—a second shot is required to arm your immune system with the power to fully fight off infection. Essentially, the first shot introduces the virus to your immune system while the second strengthens its ability to properly defend against the pathogen. If this turns about to be the necessary course of action with a coronavirus vaccine, Tan said the two shots will likely be administered about a month or two apart from each other, though that's not definitive.

Based on recent comments Fauci made expressing his concern that a vaccine isn't likely to provide long-term immunity, it's also quite possible that in addition to the two initial doses, a recurring booster shot may be necessary to maintain immunity.

While none of this is uncommon, as vaccines often have to be administered this way, it does pose potential logistical headaches for the country's healthcare system if they have to schedule and administer double doses of the vaccine to millions of people in a finite window of time. For now, we'll have to wait and see. And for more developments on COVID-19 immunity, This Is Who Could Be Last to Get the Coronavirus Vaccine, Doctors Say.

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