The national effort to immunise the UK population against COVID-19 has been phenomenal. In six months, more than 56 million vaccine doses have been given to over 36 million people. With the distribution of first and second doses going well, the British government is now considering if and when it should give people a third, booster dose.

However, reports so far have been contradictory. They’ve offered different suggestions on when boosters will be given, who they’ll be given to, and what exactly people might receive. But while it’s impossible to say for sure what will happen in the future, based on what we know about COVID-19 and the pandemic, it is possible to predict broadly how a booster programme could operate.

Undoubtedly, there’s a desire to end lockdown restrictions, reopen the country and keep it open – and this is why a third vaccine dose is being considered. Reopening safely will require two key conditions to be met: firstly, that most people get vaccinated, and secondly, that the vaccines remain effective. Boosters may be needed to ensure the latter. Here’s how they might work.

Who will be offered a third vaccine?

People whose earlier vaccine protection may have worn off are likely to be prioritised.

Currently, it’s not certain how long COVID-19 vaccine protection lasts, though studies suggest at least six months. However, vaccine protection varies by individual and naturally wanes over time. For some people, this waning happens more rapidly, for reasons such as old age, the effect of some chronic illness, or taking treatments that may suppress the immune system.

Consequently, a booster dose is likely to be needed for people in these high-risk groups to make sure good immune protection is maintained. Indeed, the UK’s Department of Health and Social Care has said that boosters will be distributed based on clinical need.

When might boosters start being given?

Ahead of when future waves of infection are anticipated. Epidemics of human coronaviruses are usually seasonal in nature, typically occurring in the winter months. Thus, the ideal timing for the vaccine booster programme will be in the autumn months so that most of the vulnerable population is protected in time for any winter outbreaks.

Current efforts are targeted at immunising the UK adult population with two doses by the autumn of 2021. It’s likely that any vaccine booster programme will only begin afterwards. Indeed, the logistics of planning, procuring the boosters, distributing them and rolling them out is fairly complex. This may delay rollout to early 2022.

Will the booster be an updated formula?

So far, the vaccines in use in the UK are showing good efficacy against circulating variants of the virus. On that basis, booster vaccines are likely to be the same as ones used previously.

However, a major concern is that a new variant emerges that can evade the immunity provided by existing vaccines – what’s known as “vaccine escape”.

Should an escape variant emerge, one strategy may be to boost people with the existing vaccine that has the best efficacy against it. For example, the B1351 variant identified in South Africa appears to have some escape potential, as the Oxford/AstraZeneca vaccine is less effective at preventing mild to moderate COVID-19 when facing it. However, early research – some of which still needs to be reviewed by other scientists – suggests the Pfizer/BioNTech vaccine isn’t so severely affected.

If none of the existing vaccines are sufficiently effective, then updated vaccines may be needed. However, they will take time to alter and mass produce. In the meantime, we need to closely monitor and study both the emergence of new variants as well as the effectiveness of existing vaccines against them.

Will I get a booster from a different manufacturer?

Possibly. Combining COVID-19 vaccines that generate immunity in different ways – such as Pfizer’s and AstraZeneca’s – could generate a more powerful and long-lasting immune response. It could also allow for greater flexibility in rolling out a third dose, which might speed things up.

However, while in theory mixing doses could give an extra boost of immunity, research is still trying to work out if this actually happens. UK scientists looking at mixing the Pfizer and AstraZeneca vaccines have so far found that side-effects are more likely when combining the two. On that basis, if an individual has reacted well to one vaccine, it may be sensible for the same vaccine to be used.

Will I need another booster in the future?

It will take more time to be sure, but again, possibly yes. This is still a new pandemic, and it’s still not known conclusively how long COVID-19 immunity lasts and how strong it will be. Based on what we know about the original Sars virus, immune responses can last years but may wane. What’s not known is whether these immune responses are sufficient to protect against severe illness or death.

It may therefore be prudent for people to be re-immunised against COVID-19 on an annual basis to maintain immunity, as many scientists believe the coronavirus will become endemic and circulate continually. If variants continue to emerge, then continually updated vaccines may become the norm, like for seasonal flu.

There’s also emerging evidence that COVID-19 vaccines don’t just protect against disease, but also reduce the probability of people spreading the virus by reducing the amount of virus infected people carry. Vaccinations could therefore be used on an ad hoc basis to quell outbreaks and emerging epidemics.

There remain many unanswered questions, and how the global pandemic will unfold in the next year is also uncertain. Vaccine policy decisions will need to be flexible and informed by emerging evidence of vaccine effectiveness and infection trends.

Andrew Lee, Reader in Global Public Health, University of Sheffield

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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