Study suggests periodic boosters to keep Covid-19 in check

Updated Covid booster shots will be important for shoring up population immunity as new variants emerge, suggests a study.

However, the trick is to target a variant for the booster that is so different from the original strain of the virus that it triggers the maturation of new and diverse antibody-producing cells, said scientists from the Washington University School of Medicine in St. Louis.

The first Covid vaccines reduced the risk of severe illness and death by more than 90 per cent. But then the virus changed. The antibodies that had worked so well against the original strain proved less effective at recognising and neutralising emerging variants, leading to breakthrough infections.

The obvious solution was to update the vaccines to target new variants, but the success of the first vaccines against the original strain made designing an effective variant booster shot tricky, said Ali Ellebedy, Associate Professor of pathology and immunology, of medicine, and of molecular microbiology at the varsity.

In the study, published in the journal Nature, the team showed that it's possible to design a variant-specific booster that doesn't just strengthen the antibodies people already have but elicits new antibodies.

"This means that periodically giving boosters targeting new variants would allow population-level protection to be maintained even as the virus evolves," Ellebedy said.

"The whole point of making boosters against new variants is to teach the immune system to recognise features in the new variants that are different from the original strain.

"But the new variants still share a lot of features with the original strain, and it's possible that the response to these shared features could dominate the response to new features. The boosters could end up just engaging immune memory cells that are already present rather than creating new memory cells, which is what we need for protection against new variants," he said.

To gauge the effectiveness of boosters at eliciting new antibodies, the researchers looked at 39 people who had received the two-shot primary sequence of the Pfizer/BioNTech or Moderna Covid-19 vaccines, followed by an experimental booster shot targeting the Beta and Delta variants.

All participants produced antibodies that neutralised the original virus strain and the Beta and Delta variants. But none of the antibodies studied were unique to Beta or Delta. The absence of such antibodies indicates that the variant booster had failed to trigger the development of detectable new antibody-producing cells, Ellebedy said.

Further, Ellebedy and team recruited eight people who had received the Pfizer/BioNTech or Moderna Covid-19 vaccine and gave them a booster targeted against the Omicron variant alone.

Studying blood samples provided by participants four months after their boosters, the researchers identified more than 300 distinct antibodies capable of neutralising the original strain, or one or more of the variants.

Of those, six neutralised Omicron but not the original strain, an indication that the booster successfully triggered the creation of new antibodies optimised for Omicron.