People who have had the Pfizer vaccine have lower antibody levels targeting the Indian Covid variant compared to other strains, new data suggests.
The research also found the levels of these antibodies are lower with increasing age and that levels decline over time.
Scientists said this is further proof of the need for booster jabs for vulnerable people this autumn.
The data, from the Francis Crick Institute and the National Institute for Health Research (NIHR) UCLH Biomedical Research Centre, also supports current plans to reduce the dose gap between the jabs.
The study found that after just one dose of the Pfizer jab, people are less likely to develop antibody levels against the Indian (B.1.617.2) variant compared to the previously dominant Kent variant (B.1.1.7).
However, levels of antibodies alone do not predict vaccine effectiveness and prospective population studies are also needed.
Lower neutralising antibody levels may still be associated with protection against Covid-19, the experts say.
The Indian variant is now believed to be dominant in the UK, with early evidence suggesting it may lead to an increased risk of being admitted to hospital compared with the Kent variant.
Cases of the Indian variant – also known as Delta – have spiked by nearly 80% from last week.
Emma Wall, an infectious diseases consultant and a research fellow for the study, said: ‘This virus will likely be around for some time to come, so we need to remain agile and vigilant.
‘Our study is designed to be responsive to shifts in the pandemic so that we can quickly provide evidence on changing risk and protection.
‘The most important thing is to ensure that vaccine protection remains high enough to keep as many people out of hospital as possible.
‘And our results suggest that the best way to do this is to quickly deliver second doses and provide boosters to those whose immunity may not be high enough against these new variants.’
Researchers have submitted their findings to the Genotype-to-Phenotype National Virology Consortium (G2P-UK), the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) and the Joint Committee on Vaccination and Immunisation (JCVI).
Eleanor Riley, professor of immunology and infectious disease, University of Edinburgh, said: ‘These data cannot tell us whether the vaccine will be any less effective at preventing severe disease, hospitalisation and death; we need to wait for the actual data on these outcomes.’
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