A nurse administers a dose of the Moderna COVID-19 vaccine during Senior Weekend at Richmond Raceway in Richmond, Va., February 2, 2021. (Parker Michels-Boyce/For the Virginia Mercury)

Nearly 10 percent of Virginians who received their first dose of COVID-19 vaccine haven’t returned for their second one, according to data from the Virginia Department of Health.

It’s a growing concern both in Virginia and nationally. The New York Times reported Sunday that more than 5 million Americans, or nearly 8 percent of all who received a first dose, have skipped their second shot. It’s more than double the rate among those vaccinated in the first few weeks of the national rollout.

In Virginia, roughly 9.4 percent of people who received a first dose of Moderna by March 7 or a first dose of Pfizer by March 14 had not returned for their second dose by April 9, according to VDH spokesman Logan Anderson. Pfizer calls for the doses to be spaced 21 days apart, while Moderna’s doses are spaced out over a month.

In total, just over 160,000 Virginians out of 1.7 million didn’t receive a second shot within the recommended time frame. 

“As the nation moves away from mass vaccination events toward more targeted outreach to specific populations and communities, it presents new challenges to be overcome,” Anderson wrote in a Wednesday email. “The bottom-line message VDH wishes to emphasize to all Virginians: Get vaccinated now — the sooner the population reaches herd immunity to stop the spread of the virus, the sooner we can return to normal life in the nation and in the commonwealth.”

According to Anderson, the department has no insight into why some Virginians aren’t following through with a second dose within the recommended time frame. But nationally, the CDC has pointed out that many people vaccinated in the early weeks of the campaign were long-term care residents and health care providers vaccinated at work, “which might have facilitated adherence to the recommended schedule.”

As vaccines become more accessible, scheduling can become more of a challenge. Many Virginians have reported that their second-dose appointment wasn’t scheduled at their first, requiring them to hunt for a second shot. In some cases, residents were turned away after trying to receive a second dose in a different location from their first.

Some residents have driven hours for their first dose and then struggled to find another appointment closer to home. The New York Times reported some people worried about the side effects sometimes associated with second doses, while others “felt that they were sufficiently protected with a single shot.”

Both Pfizer and Moderna, though — currently the most widely distributed vaccines in Virginia — require two shots for full protection. Testing for both brands showed a relatively weak immune response after the first shot, which strengthened significantly after the second. The U.S. Food and Drug Administration has explicitly warned against reducing the number of doses or lengthening the time between shots.

A growing body of evidence suggests that single doses lead to a weaker immune response, according to The New York Times, and may not provide as much protection against variants of the virus. Dr. Bill Petri, an infectious disease specialist at the University of Virginia, said recently published research indicates that a single dose can still prevent COVID-19 hospitalizations. But it drops the effectiveness of the vaccines by as much as 25 percent, and isn’t as successful at preventing transmission of the virus. 

It’s also not clear how long immunity lasts after a single shot. The good news, according to Petri, is that people can still receive a second dose — and gain more immunity — even after the recommended window has passed.

Anderson said the uptake data could prompt new challenges as the department shifts away from large vaccination clinics and toward smaller, more targeted events. Demand in many areas of the state has dropped off dramatically, and public health advocates — especially in minority communities — have called for more mobile clinics and neighborhood events.

Ideally, VDH plans to use the one-dose Johnson & Johnson vaccine in mobile clinics to remove the need for a second shot. “A mobile clinic for a single dose is just easier to set up than a mobile clinic for two doses, logistically,” Anderson said.

But the rollout of the Johnson & Johnson vaccine has hit multiple roadblocks over the last several weeks. Virginia just resumed using the vaccine after a two-week federal pause, which followed reports of extremely rare blood clotting associated with the shots. Earlier this month, a Baltimore factory contaminated and discarded 15 million doses

As a result, shipments to states dropped dramatically. Virginia received about a tenth of what health officials were originally expecting at the start of April, according to state vaccine coordinator Dr. Danny Avula.

VDH is still waiting for a significant increase in supply. A one-dose vaccine is useful in various settings, Avula has said, including efforts to immunize transient populations, like the homeless. It would also make it easier for hospitals to vaccinate patients before they’re discharged, especially in cases when they live far away.