Pfizer's vaccine worked against five coronavirus variants in the first lab study to directly compare them.
It added more evidence that both doses may be needed to protect against emerging variants.
Most people who get two doses would be protected against the variant initially from India, the researchers said.
A leading UK expert has said it's "critical" that people get two doses of Pfizer's COVID-19 vaccine to protect against emerging variants, after a lab study found the shot produced the lowest antibody response against the variant from India, especially after one dose.
Professor Deborah Dunn-Walters, chair of the British Society for Immunology COVID-19 Taskforce, said in a statement Friday that the new information in the Lancet letter showed that two doses of Pfizer's vaccine are "critical for protection" against emerging strains of the virus.
Researchers from the Francis Crick Institute and the National Institute for Health Research (NIHR) said in a letter to the Lancet medical journal on Thursday that Pfizer-BioNTechs's vaccine worked in a lab experiment to some extent against all five variants.
But the researchers found that the antibody response, a part of the immune system that fights the virus, was "significantly lower" against the Delta variant first found in India, relative to the Alpha variant first found in the UK after one dose - the variant that's been most common in the US since early April.
"Although a single dose might still afford considerably more protection than no vaccination, single-dose recipients are likely to be less protected against these SARS-CoV-2 variants," they said, referring to the Delta variant from India and the Beta variant from South Africa, which also had a relatively reduced antibody response.
The Delta variant first found in India is known to have mutations that can help it to avoid the antibody response and is more infectious.
Public Health England said on Thursday that it had overtaken the Alpha variant first found in the UK, and is now dominant in the country. In the US, the Delta variant from India accounts for less than 2% of sequenced cases in the US, according to the CDC.
Testing antibody responses
The researchers tested Pfizer's vaccine against five different variants: the original virus, an un-named variant circulating in the UK, the Delta variant first found in India, the Beta variant first found in South Africa and the Alpha variant first found in the UK.
The scientists compared the number of antibodies in the blood samples of people who'd been immunized with Pfizer's shot and administered the variants to them - 149 volunteers had received a single dose on average 30 days before, and about 159 had had two doses 28 days prior.
They also found that after two doses, the antibody response was five to eight times lower against the Delta variant first found in India, compared with the original coronavirus.
For comparison, the antibody response against the Alpha variant, first found in the UK was about two times lower, compared with the original virus, they said.
But antibodies are just one aspect of the immune system, and it's not clear from the lab tests how much protection Pfizer's vaccine will provide against each of the variants in real-life.
Why 2 doses may be key to preventing infection from emerging variants
The scientists writing to the Lancet said that they expected most people who get two doses of Pfizer's vaccine would be protected against an infection caused by the Delta variant. But it remained difficult to assess precisely to what extent the reduction in antibodies would impact how well vaccines work, they said.
Adam Finn, professor of pediatrics, University of Bristol, agreed. Finn said in a statement on Friday that the group used a newly developed lab system to measure the antibody response.
"We do not know very much about how well it predicts protection against infection or mild or severe disease. Consequently, the exact importance of these results is uncertain," he said.
The UK pioneered a strategy of delaying the length of time between doses from three weeks to 12 in December.
But, on May 16, the government brought forward second-dose appointments for Brits over 50 years-old or with underlying health conditions from 12 to 8 weeks to "ensure the strongest possible protection against the virus," as the number of infections with the Delta variant first found in India rose.
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