A new study published in the journal American Association for the Advancement of Science (AAAS) explored what would happen if the Covid-19 experimental vaccine fails to induce immune responses in vulnerable groups such as the elderly.

The researchers, Marc Lipsitch and Natalie Dean, stated in their study that in such a case an indirect protection strategy, whereby those in contact with high-risk individuals are vaccinated to reduce transmission, could be preferred once vaccine supplies become abundant.

This is in line with the influenza vaccine campaigns which initially targeted the elderly, in an effort at direct protection, but more recently have focused on the general population, in part to enhance indirect protection.

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The authors noted that measuring the indirect effects of vaccines is harder than detecting direct effects.

The researchers wrote: “It is urgent, therefore, to obtain evidence on how each candidate vaccine affects infectiousness either before approval or soon after, when scarcity may justify the randomised distribution of a vaccine. Having reliable evidence on direct and indirect protection can help plan how to use these vaccines in a coordinated way."

The researchers stated that this consideration will depend in large part on results from Phase 3 trials.

However, though, even after the trials are done, regular follow-ups will be required, including about whether approved vaccines can prevent infection or reduce contagiousness, and also subgroup-specific efficacy.

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Ideally, the Phase 3 trials in progress will identify more than one safe, effective vaccine for regulatory approval and deployment, added the researchers.

They pointed out that the clearest evidence of indirect protection from a vaccine is from one that prevents infection entirely, thereby reducing transmission.

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