Two doses of the Moderna Covid-19 vaccine are highly effective against SARS-CoV-2 variants, but protection against Delta infection decreases over time, according to a study conducted in the US.
At the time of the study, the omicron variant had not yet been detected in the Southern California region, the researchers said.
The research, published in The British Medical Journal on Wednesday, showed the vaccine was effective against Covid-19 hospitalisation associated with the Delta variant.
However, vaccine effectiveness against the variant moderately declined with increasing time after vaccination.
"This study confirmed effectiveness against all variants during the study period, although we did find a drop in effectiveness overtime against Delta infection, from 94 per cent effectiveness in the first 2 months after vaccination to 80 per cent effectiveness after six months," said Katia Bruxvoort, from Kaiser Permanente, US.
"Protection against hospitalisation due to the Delta variant remained high at 98 per cent effectiveness," said Bruxvoort, the lead author of the study.
Beginning in March 2021, Kaiser Permanente in Southern California began sending positive SARS-CoV-2 specimens from both symptomatic and asymptomatic people to a contracted laboratory for whole-genome sequencing.
This study, funded by Moderna, included 8,153 people who tested positive for SARS-CoV-2, with specimens collected from March 1 to July 27, 2021.
Among them, 91.3 per cent were unvaccinated, 1.4 per cent had received one dose of the Moderna Covid-19 vaccine, and 7.3 per cent had received two doses of the preventive.
The researchers compared people who tested positive with those who tested negative.
People who received the 2-dose Moderna vaccine were strongly protected against Covid-19 hospitalisation associated with the Delta variant, they said.
Among people who had received the 2-dose Moderna Covid-19 vaccine, no hospitalised deaths occurred, according to the researchers.
Vaccine effectiveness of two doses of Moderna Covid-19 vaccine against Delta infection was 87.9 per cent among people aged 18 to 64, and 75.2 per cent among people aged 65 and older, they said.
"While this study provides reassuring evidence of the effectiveness of 2 doses of Moderna Covid-19 vaccine in preventing Covid-19 infection and hospitalisation due to variants including Delta, it also has implications for booster shots," said Hung Fu Tseng, from Kaiser Permanente Southern California Department of Research & Evaluation.
"The findings of moderately reduced vaccine effectiveness of Moderna Covid-19 vaccine over time against Delta infection supports current booster dose recommendations,” Tseng, the study's senior author, added.
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