"The effectiveness of the vaccine against hospitalizations increases after the injection of the booster dose", reaching 85% one to two months after this second injection, according to the study published Thursday on the pre-publication site medRxiv.

The study, which has yet to be peer reviewed, was conducted by the South African Medical Research Council (SAMRC) on healthcare workers from November 15 to December 20.

Nearly half a million South African caregivers had previously received a dose of J & J's vaccine in clinical trials, then some of them subsequently received a booster dose, between six and nine months later. the first injection.

The effectiveness of the booster against Omicron was determined by comparing a group of 69,000 caregivers who received the booster, with unvaccinated individuals, in the nine provinces of the country.

Protection against hospitalization increased over time, from 63% between zero and 13 days after the booster injection, to 85% one to two months later.

"These data are important given the growing dependence (on this vaccine) in Africa," wrote the study authors.

"The effectiveness of the Johnson & Johnson anti-Covid vaccine remains strong and stable over time, including against circulating variants such as Omicron and Delta," the American laboratory said in a statement.

Earlier in December, another South African study showed that the vaccine from US laboratory Pfizer provided 70% protection against severe cases of Omicron-related disease - but with two doses.

Efficacy results from Pfizer with three doses have not yet been published.

Comparison with Pfizer

Johnson & Johnson also announced the results of a second study, involving a few dozen people first vaccinated with Pfizer, in order to compare the effect of a Johnson & Johnson booster on them compared to a third dose of Pfizer.

Two weeks after the third injection, Pfizer's booster produced more neutralizing antibodies against Omicron than J&J's booster.

But four weeks later, Pfizer's booster antibodies decreased to only a 17-fold increase, while J&J's had continued to increase, up to a 41-fold increase.

Johnson & Johnson's recall also increased the number of T cells - cells crucial in the body's defense against the virus - more than Pfizer's.

These combined results are good news for J&J, which suffered a serious blow to its image in early December, when the United States recommended to prioritize the vaccines of Pfizer and Moderna over that of Johnson & Johnson.

This US decision was based on the increased risk of thrombosis (formation of blood clots) after vaccination with Johnson & Johnson, which has caused several deaths.

The health authorities had however stressed that, in the event that no other vaccine was available, the vaccine of "J&J" always presented more benefits in the face of Covid-19 than risks.

© 2021 AFP