When the phase III trial results on Covid-19 vaccines were reported, we were delighted to find that even a single dose of vaccine was far more effective than we had dared hope. It became clear that, while the bottle-neck was vaccine availability, far more lives could be saved, and hospital and critical care admissions prevented, by providing one dose to as many people as possible, particularly those at highest risk of serious Covid-19 disease outcomes, before providing second doses. This was the basis for the UK decision to delay the second dose until 12 weeks, which has proven highly effective.

This study demonstrates – using modelling – that the same is likely to apply, not just in-country, but globally; that delaying the second dose worldwide will most quickly control the disease.

There have been concerns expressed about the lack of evidence for effectiveness if the prime-boost interval is extended by delaying the second dose. These concerns are misplaced. Everything we already knew about vaccines also tells us that a longer prime-boost interval enhances the breadth and depth of the immune response, giving longer-lasting immunity that is likely to provide greater cross-protection against variant strains.

There is relatively little knowledge about this specifically related to Covid-19 vaccines – although such data as we have seen is consistent with this. It goes beyond this paper; but it seems likely that increasing the prime-boost interval will lead to better, longer-lasting immunity, as well as protecting more people more quickly.

One of the main concerns about Covid-19 is that ‘vaccine escape mutants’ (variants of the virus which are not prevented by existing vaccines) may arise. The more viral replication there is the more likely this is to happen. It is, thus, important to control the disease as far as possible globally. If we vaccinate everybody in North America and Western Europe, but vaccine escape variants arise in unvaccinated countries, they will inevitably return to the vaccinated parts of the world, causing disease there. So we all have a vested interest in ensuring that the whole world is vaccinated as soon as possible. This paper supports that view. It raises questions about, for example, the proposed third-dose (second booster) apparently planned for the UK in Autumn 2021 – it might be in the UK’s interests, as well as global interests, for those doses to be used in countries where they are needed more.

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