However, the new data arrives just days after media reports of a troubling trend: Nearly one in 10 (8%) of Americans who got their first dose of the Pfizer or Moderna vaccines are not returning for their second shot.
“I’m very worried, because you need that second dose,” Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the U.S. Food and Drug Administration’s vaccine advisory panel, told The New York Times.
Compared with the two-dose regimen, a single shot triggers a weaker immune response and may leave some people more susceptible to dangerous virus variants. That seems to be borne out by the new study, which was led by Dr. Mark Tenforde, of the U.S. Centers for Disease Control and Prevention’s COVID-19 Response Team.
And though a single dose provides some protection against COVID-19, experts say it’s not clear how long that protection will last.
The new study tracked outcomes for 417 adults aged 65 or older who’d been admitted to one of 24 hospitals across 14 states between January and the end of March of this year.
“Patients or [their] proxies were asked about SARS-CoV-2 vaccination history, including number of doses, dates and location of vaccination,” along with any documentation confirming vaccination, Tenforde’s group said.
Besides confirming the effectiveness of two doses of the mRNA COVID-19 vaccines in keeping folks out of the hospital, the study also showed that people are not well-protected unless they are at least 14 days out from their first dose.
“There was no significant effect for receiving the first dose of a two-dose COVID-19 vaccine series within 14 days before illness onset,” the team noted.