According to the largest study of its kind, three doses of the same Covid-19 vaccine or a combination of shots works equally well to prevent infections.
Although the effectiveness of individual coronavirus vaccines is well known, the evidence for combination shots is less clear, especially for particular groups such as the elderly and those who are immunocompromised.
Now, a global analysis of data from more than 100 million people confirms that the number of doses is the key to boosting immunity rather than combinations of shots. The results were published in the BMJ.
According to the study, three doses of the same vaccine or a combination of different types work comparably to prevent Covid-19 infections, even against different variants.
Despite a rapid decline in Covid-19 infections and deaths, concerns about waning vaccine immunity and new variants make it important to understand which vaccine combinations are most effective, the BMJ said.
The Chinese University of Hong Kong (CUHK) searched 38 World Health Organization Covid-19 databases weekly to find published studies and preprints from March 8, 2022. They identified 53 studies involving 100 million people with 24 Covid vaccine course combinations.
Receiving three doses of the same vaccine is called a homologous regimen, while receiving a third dose different from those given in two primary doses is called a heterologous regimen.
After accounting for differences in study design and quality, CUHK researchers found that three doses of any mRNA vaccine appeared to be the most effective (96%) against non-serious Covid-19 infections and the most effective. effective (95%) in reducing Covid-19. related hospitalizations.
But the results also show that any three-dose regimen—heterologous or homologous—induces higher immunity in all age groups, even those over 65, than a two-dose homologous regimen.
And the study found that in immunocompromised patients, a third mRNA booster dose, as part of a heterologous or homologous regimen, also significantly improves protection compared to two doses.
The effectiveness of three-dose vaccine regimens against Covid-19-related deaths remains unclear.
The researchers acknowledged some of the limitations of the study, including that they did not assess the optimal time interval for booster or booster regimens, due to limited information.
“An mRNA boost is recommended to complement any primary vaccine course,” the researchers concluded. “Heterologous and homologous three-dose regimens work comparably to prevent Covid-19 infections, even against different variants.”