Explained: How Covid-19 vaccines fare against the Delta variant

What are the key findings?

The study found that the Delta variant was 6 times less sensitive to serum neutralising antibodies from recovered individuals, and 8 times less sensitive to vaccine-induced antibodies compared to the original Wuhan strain of the virus.

In other words, compared to the original virus, the Delta variant was 8 times more likely to cause breakthrough infections among vaccinated people, and 6 times more likely to re-infect people who have recovered from previous infections. The vaccines considered for the study were those developed by AstraZeneca and Oxford University, and Pfizer and BioNTech.

Additionally, the study reported a higher “replication and spike mediated entry” in the Delta variant, meaning it had a greater ability to infect and multiply within the human body, compared to the B.1.617.1 lineage.

The study also looked at 130 cases of breakthrough infections among fully vaccinated healthcare workers at three Delhi hospitals, and found reduced vaccine effectiveness against the Delta variant.

“The results of the study show that Delta variant does spread faster and reduces protection gained from previous infections or vaccines,” said Anurag Agrawal, director of the Delhi-based CSIR-Institute of Genomics and Integrative Biology, and a joint author of the study.

“However, the good news is that vaccination does lead to reduced severity of the disease, and so does previous infection,” he said.

What other evidence is available on the effectiveness of vaccines against Delta?

Recently, the World Health Organization cited four studies — two in the United States, one in the UK, and the other one in Qatar — that have presented similar evidence for reduced effectiveness of vaccines against the Delta variant.

The UK study, for example, showed reduced effectiveness of the AstraZeneca vaccine in a period when the Delta variant was the most dominant in the country, compared to when the Alpha variant was dominant there.

How important are vaccines, then?

Vineeta Bal, an immunologist with the Indian Institute of Science Education and Research (IISER) in Pune, pointed out that the study must not lead people to believe that vaccines were not useful. She pointed out that the Nature study was carried out on in vitro samples, in a laboratory environment.

“All data emerging from in vitro studies are surrogate evaluations in lieu of what actually happens inside the body. The limitation is that neutralising antibodies (which were tested in the study) do not provide the entire answer. Immune protection is offered by neutralising antibodies as well as the T-cell responses. In vaccinated or previously infected individuals, both antibodies and T-cells contribute to protection. This study does not show data on T-cells, thus leaving a major component of immune response out of consideration,” she said.

Bal said the results of the study were not surprising, however.

“Currently, the majority of infections are being caused by Delta variant, and it is no surprise that it is the commonest virus found in re-infection cases or in cases post vaccination,” she said.

“No vaccination provides 100% protection. Breakthrough infections are not unusual or unheard of. However, the incidence of severe disease, and hospitalisation would be significantly lower than in the vaccinated groups compared to the unvaccinated, or uninfected, groups,” she said.

Anu Raghunathan, a scientist at the National Chemical Laboratory in Pune, said the study simply means that larger amounts of antibodies would be required to block the Delta variant.

“Vaccines are still effective. The Delta variant is just less sensitive to neutralising antibodies. It means that it would require five to eight times more antibodies to elicit the same kind immune response as against the original virus during the first wave to block the Delta variant,” she said.

What is the way forward in dealing with newer variants?

The original Wuhan virus mutated into the successively more dangerous Alpha, Beta, Kappa and Delta variants. The virus will likely continue to mutate into newer forms. But all mutations need not necessarily mean they are more harmful.

Experts say the only effective way to slow down the emergence of new variants is to reduce the spread of infections through measures like vaccination, or observance of Covid-appropriate behaviour.

“Like this study, there is a critical need for continuous surveillance of the effectiveness of antibody response against new variants, and to keep assessing whether booster vaccine doses are required, or whether vaccines themselves need to be updated. Simultaneously, a genomic surveillance of new variants has to be continued,” Raghunathan said.

“This will help us in improving our vaccines and producing newer, more effective ones. In the current context, it is possible that we might require additional booster shots of vaccines. Additionally, we must ensure that when newer and more effective vaccines arrive the market, they are made accessible to everyone at a quick pace,” she said.

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