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The one-dose problem is real

We know lots of Americans skipped their second Covid-19 shot. We just don’t know exactly how many.

A person walks off a vaccination bus at a mobile vaccine clinic in New York City on December 6.
Mary Altaffer/AP

There’s a public health challenge that has been lurking and largely ignored in the US, and that could become a major issue if the omicron variant of Covid-19 becomes dominant: the one-dose problem.

There appear to be millions of Americans walking around who have received a single dose of a Covid-19 vaccine, who may think they are protected against whatever the virus can throw at them — and who could be sorely wrong.

“I’m not sure we should regard them as equivalent to unvaccinated people,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization, told me. “But they are at higher risk than fully vaccinated and boosted people.”

That was the early consensus among the experts I consulted, and the preliminary data shows, as expected, low effectiveness against omicron after one dose of the Pfizer/BioNTech vaccine. The effectiveness against omicron also declines over time after two doses but is restored to high levels (76 percent efficacy against infection) after a third dose. This was a fairly small study out of the UK, and more data will be forthcoming, but it gives an initial picture of how the vaccines are holding up against the new variant.

People who have received only one dose of a vaccine could conceivably be almost as vulnerable to infection from omicron as the unvaccinated. They may still have some level of protection against severe illness because of the multiple layers of immunity induced by the vaccines. But it’s an open question at this point — and may soon become an urgent one for the Americans who fall into this camp.

“I would not be surprised if those [with one dose] were essentially equivalent to unvaccinated when it comes to protection from infection,” Bill Hanage, a Harvard University epidemiologist, told me recently. “Other elements of the immune response might help reduce serious infections, though not to the same degree as those with more vaccination.”

Getting those people a second dose — and, eventually, a booster dose — could go a long way toward blunting the impact of omicron. The new variant appears to spread even faster than the currently dominant delta variant (itself already much quicker than the original version of the virus) and, while there is some optimism it will be somewhat milder than those previous variants, it’s also clear that full vaccination provides the best protection. Anything less than that means taking your chances with a virus that has already killed nearly 800,000 people in the United States and 5.3 million people worldwide.

But there’s at least one enormous obstacle in the way of the United States fixing this one-dose problem: We don’t know who these people are, or even how many of them are out there.

Bad data makes it harder to get patients to follow up for vaccines

The CDC vaccination data on which reporters and public health officials have relied is flawed, as Matt Yglesias wrote recently. Without more reliable data, it is really hard to accurately gauge the scale of the one-dose problem. Every expert I spoke to agrees that it exists. Nobody is sure how big the problem is, though, or for whom.

People over 65 or who are immunocompromised do not get the same protection from the full two-dose regimen as younger and healthier people. If a lot of those people didn’t even get the second dose, that poses a more serious public health problem.

“It would be helpful to have more granular data on how many people have not returned for a second dose and who they are,” Jen Kates, director of global health and HIV policy at the Kaiser Family Foundation, said.

Recognizing the flaws in data, we can still attempt to put some kind of estimate on the number of people who failed to get a second dose of either the Pfizer/BioNTech or Moderna vaccine (which are by far most prominent in the US, accounting for more than 95 percent of shots) and how serious the one-dose problem might be.

To start with the national data: There is about an 11-point gap between the share of Americans who have received at least one dose (71 percent) and the share who are fully vaccinated (60 percent). That means as many as 36 million Americans are partially but not fully vaccinated.

Some of those people are simply in between doses: the CDC recommends a three-week gap between doses for the Pfizer/BioNTech vaccine and four weeks for Moderna. But even with the current average of 500,000 Americans receiving their first dose every day — which translates to about 14 million first doses across four weeks, though the numbers are always changing — that is not nearly enough to explain the gap.

Those people are not as well protected as those with two doses. One study published in August in the New England Journal of Medicine found one dose of Pfizer’s vaccine was 30.7 percent effective against any infection from the delta variant (still dominant for now in the United States), but was 88 percent effective after two doses. Experts expect similar patterns to emerge with omicron.

How are we actually doing on getting people second doses?

As Yglesias pointed out, state and local data tends to be somewhat more reliable than the CDC’s. So I checked out the trends in Maryland and found a similar problem. The state has been averaging about 5,000 first doses per day so far in December. That would translate to as many as 140,000 people in the four-week interval between their first and second doses, if we use the longer Moderna schedule.

But the actual gap between the number of first doses versus the number of second doses? Nearly 500,000. That suggests a lot of people who got that first dose and never came back for a second.

Ohio likewise has a roughly 550,000-person gap between the number of vaccinations started and the number completed. In Washington state, a paragon of good public health practices, there are still 410,000 more people reported as having initiated their vaccination than are reported to have completed it.

None of these states are averaging anywhere near enough new vaccinations for the gap to be fully explained by people waiting the prescribed three or four weeks between shots. There is a real one-dose problem.

“I think we can consider them not fully vaccinated,” Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, told me. “People just don’t follow up because they are only so attentive to their own health or for whatever reason. Hopefully people don’t think they don’t need the second dose.”

By the historical standards set by more routine vaccines, the United States is doing pretty well with getting people their second Covid shots. For other multi-dose vaccines, research has found that as many as half of patients never show up for their additional doses. The CDC’s data has about 85 percent of people getting their second dose of the Covid-19 vaccines.

But in a pandemic, with omicron looking more transmissible and better able to evade immunity than its predecessors, any gap creates a public health problem. People probably have a variety of reasons for not getting another shot — an allergic reaction, they didn’t like the side effects, they don’t think they need it, they can’t get time off — but, in theory, they’re the low-hanging fruit for the country’s ongoing vaccination drive.

They’ve already shown a willingness to get the shot. They just need to come back to get another one. Policies like paid sick leave or programs like mobile vaccine clinics could lower the barriers for these people to finally receive their next dose, experts say.

“Someone who had bad side effects after their first dose may not get a second dose because of a lack of paid sick leave,” Rasmussen said. “Making policy that improves accessibility and ease of vaccination would make a big difference for the unvaccinated and partially vaccinated alike.”

But until the country’s vaccine data improves, finding the people to target with those efforts will be challenging — and omicron is raising the stakes of these failures.

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