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Three doses of the same Covid-19 vaccine or a combination of jabs work equally well in preventing infections, according to the largest study of its kind.

While the effectiveness of individual coronavirus vaccines is well known, the evidence around combinations of jabs has been less clear, especially for particular groups such as older people and those who are immunocompromised.

Now a global analysis of data from more than 100 million people confirms the number of doses is the key to boosting immunity rather than jab combinations. The findings have been published in the BMJ.

Three doses of either the same vaccine or a combination of different types work comparably well in preventing Covid-19 infections, even against different variants, according to the study.

Despite a rapid decline in Covid-19 infections and deaths, concerns about waning vaccine immunity and new variants makes 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 for published studies and preprints on a weekly basis from 8 March 2022. They identified 53 studies involving 100 million people with 24 combinations of Covid vaccine courses.

Receiving three doses of the same vaccine is known as a homologous regimen, while receiving a third dose that differs from those given as two primary doses is called a heterologous regimen.

After taking account of differences in study design and quality, CUHK researchers found that three doses of any mRNA vaccine appear to be most effective (96%) against non-severe Covid-19 infections and most effective (95%) in reducing Covid-19 related hospital admissions.

But the results also show that any three-dose regimen – heterologous or homologous – induces higher immunity in all age groups, even in the over-65s, 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 greatly improves protection compared with two doses.

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The effectiveness of three-dose vaccine regimens against Covid-19 related death remains uncertain.

The researchers acknowledged some of the study’s limitations, including that they didn’t evaluate the optimum time interval for prime boost or boosting regimens, owing to limited information.

“An mRNA booster is recommended to supplement any primary vaccine course,” the researchers concluded. “Heterologous and homologous three dose regimens work comparably well in preventing Covid-19 infections, even against different variants.”

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