February 28, 2024

A single dose of the Covid-19 vaccine manufactured by Johnson & Johnson is highly effective in preventing serious illness and death from the delta and beta variants of the coronavirus, data from a clinical study in South Africa suggests.

The study is the first real test of the vaccine’s effectiveness against Delta, a highly contagious variant of the virus that is emerging in the United States and much of the world. The South African Ministry of Health announced these preliminary results at a press conference on Friday. The data has not yet been peer-reviewed or published in a scientific journal.

In the study, called Sisonke, researchers evaluated a dose of the Johnson & Johnson vaccine in nearly 500,000 healthcare workers who are at high risk of Covid-19. The vaccine has up to 95 percent effectiveness against deaths from the Delta variant and up to 71 percent against hospitalizations, the researchers reported. (The vaccine did slightly worse against the beta variant, which is believed to bypass the immune response better than Delta.)

“We believe this vaccine does what it was designed to do, which is to keep people from going to the hospital and dying and dying in the intensive care unit,” said Dr. Linda-Gail Bekker, study co-lead and director of the Desmond Tutu HIV Center at the University of Cape Town.

The results suggest that people who received a dose of the Johnson & Johnson vaccine may not need a booster dose, said Dr. Bekker.

If breakthrough infections occurred in vaccinated volunteers, they resulted in mild symptoms in 96 percent of the cases and serious illness or death in less than 0.05 percent, according to the study. The process ran from February to May of this year.

The results should comfort millions of people who have received the Johnson & Johnson vaccine, especially because some previous studies suggested that the individual vaccination might be susceptible to Delta. Laboratory studies on the performance of the vaccine against the variant have been mixed, and a study comparing the one- and two-dose regimens has not published any results.

Given the uncertainty, some Johnson & Johnson recipients have requested a second dose themselves. San Francisco health officials are offering Johnson & Johnson vaccinated residents an additional dose of Pfizer BioNTech or Moderna vaccines.

South Africa approved the Johnson & Johnson vaccine in April. The country battled a surge in the beta variant of the virus earlier this year, but Delta has become the dominant variant in recent weeks. More than 8 million South Africans have received the Johnson & Johnson vaccine, or at least one dose of the Pfizer vaccine.

“This study was conducted as a real-world efficacy study in one of the most difficult epidemiological settings,” said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who led some studies for J. & J., was not involved in the trial. “This is very good news for the fight against the global Covid-19 pandemic.”

The South African researchers recorded two cases of the rare coagulation disorder associated with the Johnson & Johnson vaccine in the study; both participants made a full recovery.