The Pfizer vaccine is far less effective against the Delta variant of Covid, which is dominant in India, compared to the original strain of the coronavirus, says The Lancet journal in a new study. The antibody response to variants is even lower in people who have received just one dose and a longer gap between doses may significantly reduce antibodies against the Delta variant, the study says.
After a single dose of Pfizer, 79 per cent of people had quantifiable neutralising antibody response against the original strain, but this fell to 50 per cent for B.1.1.7 or Alpha variant, 32 per cent for Delta and 25 per cent for the B.1.351 or Beta variant first discovered in South Africa.
The researchers note it is most important to ensure that vaccine protection remains high enough to keep as many people out of hospital as possible.
"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," says Emma Wall, UCLH Infectious Diseases consultant and Senior Clinical Research Fellow for the Legacy study.
The recommendation is at odds with India's recent decision to increase the gap between two Covishield doses to 12 to 16 weeks from six-eight weeks; the government cited studies that said the effectiveness of the vaccine increased with time. Critics accused the government of widening the gap to take pressure off its vaccination drive that has been stymied by the shortage of doses and the limited supply of vaccines.
Sources in the government referenced "available real life evidence particularly from the UK" that effectiveness was significantly higher at 81.3 per cent (60.3-91.2) after two doses given at an interval of 12 weeks or longer, compared to 55.1 per cent (33-69.9) when given less than six weeks apart. That study was, however, not based on the Delta variant.
The latest Lancet study, however, supports current plans in the UK to reduce the dose gap between vaccines as it was found that after just one dose of the Pfizer-BioNTech vaccine, people were less likely to develop antibody levels against the Delta variant compared to the previously dominant Alpha variant, first found in UK's Kent.
UK's Public Health England (PHE) says experts believe the Delta variant has overtaken the Alpha strain in the country and early evidence suggests there may be an "increased risk of hospitalisation" with the Delta strain compared to the Alpha.
Lancet says Pfizer-BioNTech's vaccine produces five times fewer antibodies against the Delta variant compared to those against the original Covid strain.
The vaccine produces fewer antibodies with increasing age and the levels decline over time, Lancet says.
The team, led by researchers from the Francis Crick Institute in the UK, analysed antibodies in the blood of 250 healthy people who received either one or two doses of the Pfizer vaccine up to three months after their first dose. The researchers tested the ability of antibodies to block entry of the virus into cells – "neutralising antibodies" -- against five different variants.