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A study published Wednesday in the New England Journal of Medicine vouched for the effectiveness of the single-dose Johnson & Johnson’s Janssen COVID-19 vaccine, as US regulators continue to review reports of a very small number of recipients who were stricken with a rare blood clotting condition after getting the shot.

The study was partially funded by Janssen and coauthored by company researchers, as well as scientists from US government agencies and universities.

“Efficacy against moderate to severe–critical Covid-19 was 67% against disease with onset at least 14 days after administration and 66% against disease with onset 28 days after administration,” the study said. “Because the number of primary end-point cases was similar to the number of cases according to the FDA harmonized definition, this estimate essentially captures most of the cases of symptomatic Covid-19.”


Higher efficacy, the study continued, “against severe–critical Covid-19 was observed, with vaccine efficacy of 77% against disease with onset at least 14 days after administration and 85% against disease with onset at least 28 days after administration.”

The study comes amid a pause on administering the one-shot dose in the US, so officials can investigate a handful of cases of a rare blood-clotting condition reported among some J&J recipients, out of the roughly 7 million people who’ve received the vaccine nationwide.

And some study participants reported adverse affects from the vaccine.

“Serious adverse events, excluding those related to Covid-19, were reported by 83 of 21,895 vaccine recipients,” the study said, adding that seven “serious adverse events were considered by the investigators to be related to vaccination in the Ad26.COV2.S [vaccine] group.”

That tally included 11 recipients who experienced “venous thromboembolic events,” which are linked to the clotting issues flagged by the CDC as part of the US pause.

“Most of these participants had underlying medical conditions and predisposing factors that might have contributed to these events,” the study said. “Imbalances were also observed with regard to seizure (which occurred in 4 participants in the vaccine group vs. 1 in the placebo group) and tinnitus (in 6 vs. 0). A causal relationship between these events and Ad26.COV2.S cannot be determined.”


European regulators said Tuesday that the coronavirus vaccine, developed by the Janssen Pharmaceutical Companies of Johnson & Johnson, may be tied to very rare blood clots, but they placed no restrictions on its use in the European Union. The US pause remains in place as authorities make an independent evaluation.

New guidance is expected as early as Friday, when the CDC plans to ”convene another [Advisory Committee on Immunization Practices] meeting ... to review any additional scientific evidence,” the agency says on its website.

“Efficacy against Covid-19 involving hospitalization was 93% with regard to onset at least 14 days after administration ... and 100% with regard to onset at least 28 days after administration (no hospitalizations in the vaccine group and 16 in the placebo group),” the New England Journal study said.

The Janssen vaccine, the study authors wrote, will ultimately be a boon to the health care sector.

“The reduction in the incidence of death and the high efficacy against hospitalization are expected to substantially reduce the effect of this disease on individual persons and dramatically decrease the burden on health care systems,” the study said.

Material from the Associated Press and Washington Post was used in this report.


Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe.