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The single-dose COVID-19 vaccine candidate developed by the pharmaceutical giant Johnson & Johnson has been shown to prevent severe cases with a success rate of 85 per cent, company officials announced on Friday.

Overall, the vaccine has a 66-per-cent efficacy at preventing COVID-19, according to results from a large-scale clinical trial involving 45,000 participants in multiple locations, including the United States.

The vaccine appears to maintain its protective ability across population groups and regions, including at a study site in Africa, where many infections are due to a new South African variant of the coronavirus.

Health Canada regulators are considering the vaccine for authorization and Ottawa has already paid for 10 million doses of the vaccine with an option to purchase 28 million more.

The vaccine has lower efficacy overall than the two RNA-based vaccines already authorized by Health Canada. However, it uses a different method for alerting the immune system to the virus and is far easier to transport and deliver to large populations, particularly because it can be administered with a single shot.

The company also said that no safety issues arose during the trial.

“This vaccine is for everyone,” said Mathai Mammen, global head of research and development for Janssen Inc., the pharmaceutical arm of Johnson & Johnson. “We have … the same level of protection across all age groups, from the very young to the middle aged to the very old.”

In a separate development on Thursday, the vaccine maker Novavax, which also has a procurement deal with Canada, released results from a Phase 3 clinical trial in the U.K. showing that its vaccine prevented infections with a success rate of 89.3 per cent. Over half of the cases in the study group were caused by the U.K. variant of the virus. Based on the results of a smaller study, the company also found that the vaccine can prevent infections caused by the South African variant with a reduced efficacy of 49.4 per cent overall and 60.1 per cent in those who test negative for HIV.