The Johnson & Johnson vaccine might prevent Covid-19 infection without symptoms starting at about four weeks, according to the US Food and Drug Administration’s briefing document on the vaccine released Wednesday.
Earlier studies from the US Centers for Disease Control and Prevention have shown that most coronavirus cases are spread by people without symptoms. If a vaccine prevented asymptomatic infection, it might help reduce opportunities to transmit the disease – not just keep the vaccinated from getting sick.
The clinical trial looked for asymptomatic infections among the volunteers at several points after they had been vaccinated. The trial found that the vaccine had “modest” protection against asymptomatic infection from day one through day 29, but after that, it seemed to offer protection.
When the volunteers were checked at 71 days after they had gotten the vaccine, it seemed about 74% effective against asymptomatic infections.
The document says these findings should be “interpreted with caution” and additional research is needed, since the follow-up time is limited and the number of volunteers in this sample was comparatively small, so “definitive conclusions cannot be drawn at this time.”