New studies show that one shot of a vaccine can greatly amplify antibody levels in those who have recovered from the coronavirus.

In another new study, researchers at New York University found that a second dose of the vaccine did not add much benefit at all for people who have had Covid-19 — a phenomenon that has also been observed with vaccines for other viruses.

In that study, most people had been infected with the coronavirus eight or nine months earlier, but saw their antibodies increase by a hundredfold to a thousandfold when given the first dose of a vaccine. After the second dose, however, the antibody levels did not increase any further.

Dr. Krammer led another of the new studies, which showed that people who have had Covid-19 and received one dose of a vaccine experienced more severe side effects from the inoculation and had more antibodies compared with those who had not been infected before.

“If you put all four papers together, that’s providing pretty good information about people who already had an infection only needing one vaccination,” Dr. Krammer said.

Johnson & Johnson plans to test its coronavirus vaccine in infants and even in newborns, as well as in pregnant women and in people who have compromised immune systems.

The bold plan for expanded clinical trials met with the approval of Dr. Ofer Levy, director of the Precision Vaccines Program at Harvard’s Boston Children’s Hospital and a member of the Food and Drug Administration advisory committee that reviewed the company’s vaccine data.

When Dr. Levy saw the outlines of the planned trials, “they turned my head,” he said. They were reported as part of the company’s application to the F.D.A. for emergency use approval and discussed at the F.D.A. meeting.

“They did not get into a lot of detail about it but did make it clear they will be pursuing pediatric and maternal coronavirus immunization studies,” Dr. Levy said. They referred committee members to their briefing materials where, on page 34, the company mentioned the planned studies.

https://www.nytimes.com/2021/02/28/world/johnson-and-johnson-vaccine-testing-children.html