Delta has been declared as Variant of Concern (VOC) by the World Health Organisation (WHO). First detected in India, the Delta variant is now dominant in the UK. It has displaced the B.1.1.7 (Alpha) strain 2 that emerged in the UK with the second Covid wave in late 2020.

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The study said that the efficacy of Covid vaccines against B.1.617.2 was unknown so it carried out an initial analysis to track serological responses to vaccination.  

The study was conducted using Pfizer–BioNTech vaccine against five Covid strains, including variants of concerns B.1.617.2 (Delta) and B.1.351 (Beta) first detected in South Africa). The other three variants were: a strain with the original spike sequence (Wild-type); a strain with an Asp614Gly mutation isolated during the first wave of infection in the UK (D614G); and B.1.1.7 (Alpha). 

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The study found that two doses of Pfizer vaccine “elicited ELISA-detected anti-Wild-type spike antibodies” in all participants, and neutralising antibodies against all strains, including the three VOCs. However, six (3%) and nine (5%) of 159 participants lacked antibodies against B.1.617.2 and B.1.351, respectively.  

The study also compared the vaccine efficacy between the two doses.

According to the study, neutralisation of variants of concern was “markedly different” after only one dose of Pfizer vaccine. It also said that most participants who received two doses of Pfizer vaccine “would be protected against B.1.617.2 infection”. 

“In the case of two BNT162b2 (Pfizer) doses, our cohort of generally healthy, relatively young, recently vaccinated, and mostly single-ethnicity individuals presents a reasonable best-case scenario for NAb (neutralising antibodies) activity against SARS-CoV-2 variants,” the study said. It, however, added that regardless of the absolute vaccine efficacy requirements, “peak NAbTs (NAb titres) are significantly reduced against VOCs B.1.617.2 and B.1.351 compared with NAbTs against earlier variants”.

In the case of single-dose recipients, the study said, data showed that NAbTs were significantly lower against Delta and Beta variants relative to Alpha. “Single-dose recipients are likely to be less protected against these SARS-CoV-2 variants,” it said. 

“These data therefore suggest that the benefits of delaying the second dose, in terms of wider population coverage and increased individual NAbTs after the second dose, must now be weighed against decreased efficacy in the short-term, in the context of the spread of B.1.617.2,” the study said.

Earlier, the UK had extended the interval between the two doses to maximise population coverage. 

The study said that its data highlighted the need to increase vaccine supply to allow all countries “to extend second-dose protection as quickly as possible”.

On May 13, India extended the gap between the first and second doses of Covishield vaccine, manufactured by Serum Institute, to 12-16 weeks. Earlier, the gap was 6-8 weeks.