Almost two years have passed since Margaret Keenan first person in the world to receive an approved COVID vaccine at a clinic in Coventry on 8 December 2020. Since almost 13 billion doses of various COVID vaccines have been administered worldwide. And I think they did prevent millions of deaths.

The UK government reports that almost 54 million Britons received their first dose, almost 51 million received a second dose and more than 40 million received a third dose or booster. The NHS recently launched an Autumn boost campaign with a fourth dose for more vulnerable groups.

Fall boosters are available for those over 50 or with certain health conditions, as well as frontline health and social workers to maximize their protection for the winter. More than half of the 26 million people in England who qualify for an Autumn Booster have now taken advantage of it.

Although prioritizing risk groups was the right approach when introducing booster vaccinations, would it be worthwhile to extend the offer to adults under 50 in the UK at this stage? Data is not yet conclusive on how effective it will be. However, the available evidence suggests that the benefits of a fourth dose outweigh the risks for younger adults.

Like the United Kingdom, some countries have implemented targeted booster campaigns that make the fourth dose available only to vulnerable groups. For example, France, Italy, Germany and Israel offer a fourth dose to adults over 60 or with underlying medical conditions, while the US offers second booster shots to those over 50 or with a compromised immune system.

On the other hand, Australia not only recommends a fourth dose for adults over 50, but has also recommended that anyone between the ages of 30 and 49 can get one if they want.

At the start of the pandemic, vaccine supplies struggled to keep up with the massive demand, justifying rationing and strict strategic planning of allocating available doses. But vaccine shortages have become less of a problem in the UK as demand has fallen.

With this in mind, and as different countries take different approaches to qualifying for a second overhaul, it is reasonable to question the best course of action.

What does the evidence say?

Although there is considerable evidence that fourth doses are effective in boosting immune responses and reducing the risk of death in the elderly and debilitated, we have limited data on which to base the decision to offer them to healthy younger adults.

A study published in March 2022 found that the fourth dose of either Pfizer or Moderna mRNA vaccines restored antibody levels in healthcare workers over the age of 29. Compared to a control group that had only three doses, those that had four doses were slightly better protected against infection and symptomatic disease. However, the authors noted that “a fourth vaccination of healthy young health workers may have only modest benefits”.

Another study, published in July 2022, looked at antibodies after a fourth dose in a group of health workers with an average age of 43 years. Based on the results, the authors suggested that a fourth dose could prevent severe cases of COVID and reduce hospitalizations in younger and healthier people.

Both studies reported only mild side effects, such as pain at the injection site, after more extensive evidence showed the safety of the fourth dose.

a ‘Catch 22’

At a time when the majority of the world’s population has developed some immunity to COVID (via vaccination, infection, or both), the potentially limited additional protection afforded to young adults by second booster shots justifies the logistical and financial costs of justify their administration. ?

The limited availability of evidence to determine whether the fourth dose should be offered to younger people represents a catch-22 of sorts. We need more data to answer this question with certainty. At the same time, not much additional data can be obtained without allowing healthy young adults to receive the fourth vaccination if they so choose.

However, based on the available evidence, there are two main factors that together suggest that it may be worthwhile to extend the fourth vaccine offer to healthy people under the age of 50.

The first is that immunity to COVID (acquired by vaccination or infection) declines over time, especially with respect to new virus variants. The vaccines, given as a fourth dose in the UK and elsewhere, are known as bivalent boosters, which aim to provide broader protection by providing immunity against both the original strain of the virus and the Omicron variant strengthen.

The second important factor is that the side effects of COVID vaccinations are rarer and more severe than the well-documented effects of the infection itself, many of which are long-lasting and not yet fully understood.

Overall, as we continue to gather and analyze evidence, it may make sense to extend the offer of a fourth dose on a voluntary basis to all adults under the age of 50. If someone in their 30s whose day job is teaching large groups of people, if I got a fourth dose tomorrow I would not hesitate to take it to protect myself and my students.

Alessandro Siani, Deputy Head (Students), School of Biological Sciences, University of Portsmouth

This article has been republished by The Conversation under a Creative Commons license. Read the original article.