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.