Johnson & Johnson’s COVID-19 vaccine cuts the risk of getting infected with the disease by about half, according to the latest results of a trial involving almost half a million health workers in South Africa.
The vast majority of the breakthrough infections were mild, Glenda Gray, co-leader of the study known as Sisonke, said in an interview, citing unpublished data from the trial, which had earlier shown the shot’s effectiveness against severe illness.
Like all COVID vaccines, J&J’s was intended and tested for its ability to prevent COVID hospitalizations and deaths. Even so, the frequency of breakthrough infections in vaccinated people highlights the challenge governments face in halting the virus’s spread, which threatens to lead to the proliferation of new variants that may be even more contagious.
Coupled with vaccine hesitancy, the limited efficacy of shots in stopping mild infections will mean “we will continue to see a flow of infections,” said Bruce Mellado, a professor at the University of the Witwatersrand in Johannesburg who uses modelling to predict the trajectory of infections. Still, he said, the effectiveness against death and severe disease could prevent a “human catastrophe.”
J&J didn’t immediately respond to a request for comment.
The study included several weeks during which South Africa was going through a third wave of coronavirus infections, spurred by the highly contagious delta variant. The emergence of new, fast-spreading strains has made earlier goals of herd immunity – when the proportion of the population that’s vulnerable to the virus sinks so low that it stifles spread – harder to reach.
By cutting the number and the intensity of infections, the vaccines limit the chance of further mutated strains emerging. Yet for many countries, the focus has shifted to reducing the seriousness of illness and subsequent demand for more intensive treatments.