Millions of Americans who have received one dose of either the Moderna or Pfizer/BioNTech Covid-19 vaccines have missed their second doses, according to new CDC data—a finding that has public health officials and experts urging the partially vaccinated to get their second shot.

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'I'm very worried'

According to the New York Times, CDC data indicates that as of April 9, 8% of Americans who've received one dose of either of the two two-dose vaccines currently authorized for use in the country have skipped their second doses. That’s twice the rate of people who were vaccinated earlier in the pandemic. The data includes only people who got a first Moderna shot by March 7 or an initial Pfizer/BioNTech shot by March 14, the Times reports.

According to CDC research, the first shots from both the Pfizer/BioNTech and Moderna vaccines provide about 80% protection from coronavirus infection. That number increases to 90% to 95% around two weeks after the second dose for both vaccines.

However, the agency said there is limited evidence available on how effective the currently authorized vaccines are if separated by more than six weeks, although other nations—such as Britain and Canada—are spacing out doses by as much as three or four months.

Moreover, according to the Times, burgeoning evidence suggests that while a single dose provides some protection against the coronavirus, it triggers a weaker immune response than a full, two-shot dose and could leave the partially vaccinated more vulnerable to more contagious variants. It's also unclear, the Times reports, how long the protection afforded by just one dose will last.

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"I'm very worried, because you need that second dose," Paul Offit, a professor at the University of Pennsylvania and member of FDA's vaccination advisory panel, said.

That said, according to the Times, overall follow-through for second doses remains high, with about 92% of those who receive the first shot receiving the second. In comparison, follow-through for the second dose of the shingles vaccines is about 75%.

Why Americans are missing their second doses

Currently, the only people who are advised to forgo a second dose are those who have experienced allergic reactions to the first, the Times reports. However, many who talked with the Times cited other reasons for opting against their second shot, including concerns about side effects, the belief that one shot provided sufficient protection against Covid-19, and—among many respondents—logistical challenges accessing the second dose, such as vaccination sites canceling or rescheduling second-dose appointments after running out of supply.

According to Elena Cyrus, an infectious disease epidemiologist at the University of Central Florida, while some people are able to be flexible about having their appointment rescheduled, others who don't have access to reliable transportation or have a job with very specific hours could have a harder time rebooking their appointment.

College students also pose a unique challenge, as many recently became eligible to receive a vaccine and as a result, are getting their first shots on campus. But when the time comes for their second doses, many of students won't be on campus anymore, the Times reports.

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In response to the growing trend, public health officials across the country are trying to reach the partially vaccinated to encourage full vaccination, according to the Times. For instance, officials in Arkansas and Illinois have deployed teams to call, text, or write to partially vaccinated people to remind them about their second doses. In Pennsylvania, officials are making plans to ensure college students who have gotten one shot on campus will be able to access the second easily over summer vacation. And officials in South Carolina have allocated a few thousand doses to go to people overdue for their second dose. 

First dose provides 'head start' for immune system

While there's no clear data on how many people have contracted Covid-19 between their first and second doses of a coronavirus vaccine, an analysis from the Washington Post estimated that around 21,000 of 470,000 people who tested positive for Covid-19 last week already received the first dose of their vaccine.

The Post specified that these are infections that occurred before the immune system was able to take full effect, rather than so-called "breakthrough infections" that occur more than two weeks after receiving both shots. Also, cases among partially vaccinated people may have occurred before the individual even got his or her first shot—only presenting afterwards—or might have involved exposure to the virus before protection from the first dose had kicked in, which typically takes at least a week.

While evidence indicates that being fully vaccinated provides more protection than being partially vaccinated, experts said the first dose alone is still likely to prevent a severe case of Covid-19. "Even if you develop disease, you already have a head start from an immune system standpoint on controlling the virus," C. Buddy Creech, director of Vanderbilt University's vaccine research program, said. "The real challenge is we have to show the blueprint to the immune system with enough lead time."

Separately, Krutika Kuppalli, an infectious disease doctor in South Carolina, said most of the patients she treated who had tested positive for Covid-19 after receiving an initial vaccine dose had comparatively mild symptoms. "The thing people need to remember is the vaccine is not 100% protective, nothing is 100% protective," she said. "We want this to become akin to it feeling like a nuisance cold if you get vaccinated. We don't want people having significant morbidity and mortality from [Covid-19]" (Robbins, New York Times, 4/26; Choi, The Hill, 4/25 [1]; Nirappil/Keating, Washington Post, 4/24; Choi, The Hill, 4/25 [2]).