This week's roundup of some of the latest scientific studies on the coronavirus and efforts to find treatments and vaccines for COVID-19 revisits concerns about vaccine dosing and efficacy and raises hopes that one dose may be enough in certain cases.

One dose might be enough for COVID-19 survivors

COVID-19 survivors might only need one shot of the new vaccines from Moderna Inc. and Pfizer/BioNTech, instead of the usual two doses, because their immune systems have gotten a head start on learning to recognize the virus, according to two separate reports posted on medRxiv ahead of peer review.

In one study of 59 health care workers who recovered from COVID-19 and received one of the vaccines, antibody levels after the first shot were higher than levels usually seen after two doses in people without a history of COVID-19. In a separate study, researchers found that 41 COVID-19 survivors developed "high antibody titers within days of vaccination," and those levels were 10 to 20 times higher than in uninfected, unvaccinated volunteers after just one vaccine dose.

"The antibody response to the first vaccine dose in individuals with pre-existing immunity is equal to or even exceeds" levels found in uninfected individuals after the second vaccine dose, the authors of that paper said. "Changing the policy to give these individuals only one dose of vaccine would not negatively impact their antibody titers, spare them from unnecessary pain and free up many urgently needed vaccine doses," they said.

Oxford/AstraZeneca vaccine might work better with doses months apart

Among recipients of the COVID-19 vaccine from Oxford University and AstraZeneca, prolonging the interval between the first and second doses led to better results, researchers said in a paper posted last week ahead of a peer review by The Lancet on its preprint site.

For volunteers aged 18 to 55, vaccine efficacy was 82.4% with 12 or more weeks between doses, compared to 54.9% when the booster was given within six weeks after the first dose. The longest interval between doses given to older volunteers was eight weeks, so there was no data for the efficacy of a 12-week dosing gap in that group.

Europe's medicine regulator has said there is not enough data to determine how well the vaccine will work in people over 55. Given their findings, the authors say, "A second dose given after a three-month period is an effective strategy ... and may be the optimal for rollout of a pandemic vaccine when supplies are limited in the short term."

Delaying the second dose of the COVID-19 vaccine has been a contested issue in the medical community.

J&J vaccine 66% effective in global trial

Johnson & Johnson's single-dose vaccine was 66% effective in preventing moderate and severe COVID-19 in a late-stage global trial with nearly 44,000 volunteers that includes regions with worrisome variants of the virus, the company said.

Protection levels 28 days after vaccination varied from 72% in the United States to 66% in Latin America and just 57% in South Africa, where a potent new variant has become prevalent. Two-dose vaccines from Pfizer /BioNTech and Moderna were around 95% effective in pivotal trials, but those were conducted before the highly transmissible new variants emerged.

"Right now, any protection and additional vaccine is great," said Walid Gellad, a health policy associate professor at the University of Pittsburgh who was not involved in the trial. "The key is not only overall efficacy but specifically efficacy against severe disease, hospitalization and death."

In a news release, J&J said its vaccine was 85% effective in stopping severe disease and preventing hospitalization across all geographies and against multiple variants. The vaccine uses a common cold virus to introduce coronavirus proteins into cells and trigger an immune response. The company plans to seek emergency use authorization from the U.S. Food and Drug Administration (FDA) next week.