Covid-19 vaccines remain effective six months after second dose

Two doses of Covid-19 vaccines were sufficient in providing protection against severe cases of the infectious disease, according to new research. 

A studyled by researchers at the University of Bristol found that two doses of Pfizer-BioNTech and AstraZeneca Covid-19 vaccines provided protection against Covid-19 infections six months after the second dose.  

The study investigated how quickly vaccine effectiveness reduced over time in adults who hadn’t already been infected with SARS-CoV-2 and who had received two doses of the Pfizer-BioNTech or AstraZeneca Covid-19 vaccine compared with unvaccinated individuals. 

The researchers used NHS health records from over 7 million adults to assess vaccine effectiveness. They found that rates of Covid-19 associated hospital admission and deaths were substantially lower among vaccinated than unvaccinated adults up to six months after their second dose.  

More so, the Pfizer-BioNTech vaccine and AstraZeneca vaccines were 80% & 75% effective at reducing hospital admission and death linked to Covid-19. However, waning vaccine effectiveness against Covid-19 meant that rates in vaccinated individuals were similar to or higher than in unvaccinated individuals by six months after the second dose. 

The team now aims to lead a follow-up study looking at vaccine effectiveness at one-year following a second date, also taking into account the Omicron variant and vulnerable groups.    

Official comments

Dr Elsie Horne, Senior Research Associate in Medical Statistics and Health Data Science in Bristol Medical School: Population Health Sciences (PHS) and the study’s lead author, said: “Until now there has been limited and conflicting evidence relating to the rate of waning following second dose of Covid-19 vaccines, whether it extends to severe Covid-19, and whether the rate differs according to age and clinical vulnerability.  

“Although we found that protection against severe Covid-19 provided by two doses of vaccine wanes over time, the very high initial protection means that, despite waning, protection remains high six months after the second dose. This finding was consistent across all adults, including older adults and those who are at risk of severe Covid-19.” 

Jonathan Sterne, Professor of Medical Statistics and Epidemiology in Bristol Medical School and co-author of the study added: “We found that the rate at which vaccine effectiveness waned was consistent across subgroups defined by age and clinical vulnerability. Studying how long Covid-19 vaccines remain effective continues to be important to scheduling and targeting of booster vaccinations.” 



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