New efficacy data from Brazil on Sinovac’s Covid-19 vaccine, for which there are now four widely different success rates, raised more questions than answers, as many developing countries pin their hopes on the CoronaVac to end the pandemic.

Trial size, patient criteria, duration of post-vaccination observation, target groups, prevalence of the virus in each site are among the factors that could impact efficacy, experts said.

More rampant infections in Brazil than in other trial sites and its study design to focus on medical workers could have led to more mild infections in trials, thus lowering efficacy data, a person familiar with the matter said.

The new lower efficacy finding included people with very mild symptoms who did not need clinical assistance, while the earlier reading was based on patients with symptoms that needed some assistance, Brazilian researchers said.

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