Gamma Variant Vulnerable to Vaccine

The AstraZeneca vaccine, widely used in Brazil and elsewhere in response to a wave of infections from the SARS-CoV-2 gamma variant, affords significant protection to older populations when the vaccine’s full two-dose schedule is completed.

Research by the Yale School of Public Health and a host of Brazilian and international scientists analyzed how adults aged 60 and older in São Paulo responded to either just one dose of the vaccine or the full two-dose regimen during extensive transmission of the gamma variant (also known as P.1 or the “Brazilian variant”). This variant has caused widespread sickness and death since it emerged in Brazil last year and has subsequently spread elsewhere in South American and beyond.

“These findings have major implications for policy in many countries that have spaced out vaccination and especially those in South America that are undergoing large gamma-associated epidemics,” said Julio Croda, M.D., Ph.D., adjunct associate professor at the Yale School of Public Health, a researcher at the Oswaldo Cruz Foundation in Brazil and the study’s principal investigator. Known as ChAdOx1, the AstraZeneca vaccine is used worldwide for COVID-19.

The researchers found that a single dose of the vaccine among the older adults conferred protection against COVID-19, but its effectiveness was modest. One dose had an overall effectiveness of 33% against symptomatic illness and reduced hospitalizations by 55% and deaths by 61%.

In contrast, a second dose of the AstraZeneca vaccine was found to provide significantly higher protection. A complete two-dose regimen provided an overall effectiveness of 78% against COVID-19 and reduced hospitalizations by 88% and deaths by 94%.

“The good news is that this vaccine was highly effective in protecting the elderly during an epidemic where the gamma variant caused more than 80% of the cases,” said Albert Ko, M.D., the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health and the study’s co-principal investigator. “However, unlike the experience with ChAdOx1 in other settings, two doses are needed to reach optimal levels of protection in this vulnerable population and in the setting of extensive gamma variant transmission.”