Three doses of same or mixed jabs work equally well against Covid

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

The findings, published in The BMJ, suggest that the number of vaccine doses seems to be the key to improving immunity rather than the combinations of vaccine types, and should help inform future public health decision making.

While the effectiveness of individual vaccines for Covid-19 are well known, the effectiveness of vaccine combinations is less clear, especially for particular groups, such as older people and those who are immunocompromised, the researchers said.

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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, they said.

The researchers from the Chinese University of Hong Kong (CUHK) analysed 38 World Health Organisation (WHO) Covid-19 databases for published studies and preprints on a weekly basis from March 8, 2022.

They identified 53 studies involving over 100 million participants with 24 combinations of approved Covid-19 vaccine courses and seven different vaccine types for analysis.

"While a three dose mRNA regimen seems to be the most effective in preventing Covid-19 infections, any heterologous and homologous three dose regimens work comparably well in preventing Covid-19 infections, even against different variants," the authors of the study said.

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

The researchers found that three doses of any mRNA vaccine appear to be most effective (96 per cent) against non-severe Covid-19 infections and most effective (95 per cent) in reducing Covid-19 related hospital admission.

Using an mRNA booster after two doses of adenovirus vector vaccines also has a satisfactory effectiveness of 88 per cent, they said.

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, according to the researchers.

A third booster dose is needed to prevent infection caused by the Omicron variant, they said.

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The study found that in immunocompromised patients, a third mRNA booster dose, as part of a heterologous or homologous regimen, greatly improves protection compared with two doses.

However, the effectiveness of three dose vaccine regimens against Covid-19 related death remains uncertain, the findings show.

These are statistical analyses of observational and randomised controlled trial findings.

The researchers acknowledge that they did not evaluate the optimum time interval for prime boost or boosting regimens, owing to limited information.