People fully vaccinated with two doses of the Pfizer-BioNTech vaccine are likely to have more than five times lower levels of neutralising antibodies against the Delta variant (B16172) when compared to the original strain, according to new laboratory data that supports Pfizer's plans to deliver booster shots in Autumn.

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This antibody's response was even lower in people who had only received one dose. After a single dose of Pfizer-BioNTech, 79 per cent of people had a quantifiable neutralising antibody response against the original strain, but this fell to 50 per cent for Alpha variant (B117), 32 per cent for Delta variant (B16172) and 25 per cent for Beta variant (B1351), showed the study, published as a Research Letter in The Lancet on 3 June.

The results also show that levels of these antibodies are lower with increasing age and that levels decline over time, while no correlation was observed for sex or body mass index. Although laboratory results such as these are needed to provide a guide as to how the virus might be evolving to escape the first generation of vaccines, 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, said researchers from the Francis Crick Institute and the National Institute for Health Research (NIHR) UCLH Biomedical Research Centre in the UK.

"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", said Emma Wall, UCLH Infectious Diseases consultant. "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", she added.

The findings also support current plans to reduce the dose gap between vaccines since they found that after just one dose of the Pfizer-BioNTech vaccine, people are less likely to develop antibody levels against the Delta variant as high as those seen against the previously dominant Alpha (B117) variant. For the study, the team analysed antibodies in the blood of 250 healthy people who received either one or two doses of the Pfizer-BioNTech COVID-19 vaccine, up to three months after their first dose.

The researchers tested the ability of antibodies to block entry of the virus into cells, so-called 'neutralising antibodies', against the original strain first discovered in Wuhan, China; the dominant strain in Europe during the first wave in April 2020 (D614G); and the Alpha, Beta and Delta variants. The study will be extended to participants vaccinated with the Oxford/AstraZeneca vaccine.

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), as evidence of the level of protection people might receive against the new variants after one dose and both doses of the Pfizer COVID-19 vaccine.