SALT LAKE CITY — As of Thursday, 714,049 Utahns have been vaccinated against COVID-19. But of those, only 394,004 are fully vaccinated.
That means 320,045 Utahns still have another shot to go, provided they received one of the vaccines developed by Pfizer-BioNTech or by Moderna; both require two doses for maximum effect. A third vaccine developed by Johnson & Johnson, which was recently approved for use, requires only one dose.
An AstraZeneca vaccine, likely the next approved for widespread use in America, requires two doses as well.
But how much does it really matter to get the second dose of the vaccine? After all, scientists agree that getting one dose of the Pfizer and Moderna vaccine will create some degree of immunity, introducing the body to a virus it was previously unfamiliar with and starting the process of antibody creation.
Two doctors KSL.com spoke to Thursday urged Utahns to finish their course of the vaccine, though, saying it's crucial for the state to develop the herd immunity it seeks before lifting all coronavirus-related restrictions.
'Nearly 100%' effectiveness
Dr. Tamara Sheffield, medical director of community health and prevention for Intermountain Healthcare, said it's not uncommon for vaccines of all kinds to require more than one dose.
"Most vaccines will have multiple doses, in a series, in order to stimulate the immune response," Sheffield said. "The first year a child gets the influenza vaccine, they need two doses because they have not been exposed to the influenza vaccine in the past." It's only after that when patients get their yearly flu shot that they only need one dose at a time, Sheffield added.
The second dose of the coronavirus vaccine creates a greater, more effective immune response and also makes that response last longer. It's humans' "memory T" cells, Sheffield said, that create long-lasting virus prevention and may require more than one exposure to activate.
Estimates vary, but the Pfizer vaccine's effectiveness after one dose is believed to be about 52%, based on its own data. After the second dose, that jumps to roughly 95% — a nearly unprecedented level of protection for modern vaccines, doctors say.
Dr. Emily Spivak, infectious disease physician at University of Utah Health, said the full vaccines are "nearly 100%" effective at preventing severe cases of the disease.
"People sort of get in the weeds of, well, this one is 70% effective ... versus 90%," Spivak said. But they all prevent severe effects of the disease almost entirely, and that's the important part, she said.
"I think regardless of the numbers, the real take-home is that you can get COVID-19 — and I have seen it numerous times — between your first and second dose," Spivak said. She encouraged Utahns to stay vigilant even a week or two after they've received their second dose.
"I hesitate for people to obsess about the actual number — is it 70, or is it 50? — because the answer is, the two doses make it extremely effective. Like, never-seen-before effective, except maybe for measles."
Officials recommend that the two doses of the Pfizer-BioNTech vaccine are taken 21 days apart and that Moderna doses are taken 28 days apart. Sheffield and Spivak said it probably doesn't hurt to wait a little bit longer than that, but recommend that Utahns avoid getting a second dose too soon.
Mutations and asymptomatic spread
Sheffield said there are two other big reasons why Utahns should get their second dose.
The first relates to herd immunity. People who are only partially vaccinated and partially protected, she said, might become unwitting vectors of virus transmission. They could be protected against symptoms, Sheffield said, while still passing the coronavirus to others.
"When you're partially protected, you may still have the disease, still be able to pass it, but not maybe feel the symptoms of it," she said. "So you may be more likely to be a spreader."
Partially vaccinated individuals are also more likely to create new virus mutations, Sheffield explained, like the ones that originated in Brazil, South Africa and the U.K.
Getting only one dose of a two-shot vaccine creates a "partial or weak" immune response, similar to what happens with an immunocompromised individual, Sheffield said.
"Those individuals who have that weaker immune response tend to be the ones where we get escaped mutations," she said. "Viruses that mutate easily, like coronavirus does — a partially immune individual is one where those mutations are more likely to survive, because they're different and you're immune system doesn't catch them. So they proliferate.
"While the regular virus you might be able to control, the mutated virus you don't control as well."
For those reasons and many more, Spivak pleaded with Utahns to get their second vaccine dose if necessary. "Please, please, definitely stick with the recommendations," she said. "Get your second dose."