As more positive results from clinical trials of vaccines are announced, countries must consider how they can introduce immunisation programmes.
They face a tough choice: start with the elderly and vulnerable, who have high mortality rates but often minimal social interactions.
Or inoculate younger more socially active people – and people in their 20s, 30s and 40s who make up the bulk of the workforce.
Vaccine stockpiles have already been built up but supplies will be limited in the coming months.
Some nations, such as the UK, are prioritising primarily by age, in line with the pattern that mortality rates increase steeply as people get older.
Greatest priority is given to older adults in care homes and care home workers, followed by those over 80 and healthcare and social care workers.
It is only when all over-65s have been offered the vaccine that high-risk adults younger than 65 will be included, which could be mid-2021 or later.
According to David Taylor, professor emeritus of pharmaceutical and public health policy at University College London, it is “a sensible way forward” to focus on those at raised risk who can safely be given a vaccine.
“The easiest way to do that is by age. I think it’s a reasonable strategy,” he said.
While there are detail differences among other major European countries, such as Germany, Italy and Spain, they too are also focusing on those most vulnerable because of health conditions or age, plus healthcare workers.
France’s approach is similar but casts a wider net when it comes to people at increased risk because of their occupation.
Their prioritised list includes 23 million people who are elderly or have underlying medical conditions, plus 1.8 million workers in healthcare and related sectors, and 5 million taxi drivers, shop workers, school employees and others who have frequent contact with the public. Taxi drivers are reported to have suffered high mortality early on in the pandemic.
The US Centres for Disease Control suggests that, along with other at-risk groups, members of ethnic minority communities could be prioritised because they have been hit harder by the virus.
Latest research from Khalifa University, however, appears to upend conventional wisdom when it comes to whom vaccination programmes should focus on.
Computer modelling suggests priority should be given to people who have most interactions with others, typically younger individuals, even if they themselves are unlikely to fall seriously ill should they contract the coronavirus.
Delicate balancing act for decision-makers
Under one scenario modelled by the researchers (involving 75 per cent vaccine effectiveness and 80 per cent coverage), prioritising according to the number of interactions a person has cut deaths by 63.5 per cent compared to vaccination without any prioritisation. A refinement of this strategy reduced fatalities by 71.7 per cent.
“The criteria for groups' vaccination priority should not be those with the highest mortality but rather those the highest number of daily person-to-person interactions,” the authors wrote.
The rationale is that vaccinating people who are most likely to interact with others does not only reduce mortality within the vaccinated group (who are at modest risk), but also cuts the number of other people who will be infected by members of this group. This is called the “amplification effect”.
Notably, the results indicate that prioritising those with greatest mortality – the strategy most governments are adopting – results in more deaths than a vaccination programme that does not prioritise any groups. With 75 per cent vaccine effectiveness and 80 per cent coverage, modelling found this approach resulted in an 11.6 per cent increase in deaths compared to no prioritisation.
Published in medRxiv, the findings have yet to be viewed by outside researchers.
World's younger people may 'shun vaccine shots'
Another advantage of focusing initially on younger people may be speed, which could be particularly important in vaccination programmes of the scale being envisaged.
Dr Bharat Pankhania, a senior clinical lecturer at the University of Exeter in the UK and consultant in communicable disease control, said that, from his experience in clinics, dealing with older people who are being vaccinated takes longer, potentially reducing throughput.
“It takes time. In an emergency, if you can immunise as many people as you can, it’s a good idea,” he said.
But he said that prioritising younger people “wouldn’t go down very well”, so a strategy that focuses on “the elderly, the spreaders and the healthcare workers” was likely to be better.
A possible drawback of focusing on younger people may be that they will be less likely to take up the offer of a vaccine. Compulsory vaccination would get around this, but few governments are keen on the idea.
“People at known risk would be very quick to take the vaccine offered, but those with little risk would probably not. Why should they bother if they are not likely to get sick?” said Ian Jones, a professor of virology at the University of Reading in the UK.
Another issue Prof Jones highlighted was the limitations of vaccine supplies, which mean its introduction would have to be staggered, “leaving open the transmission routes for some time” with the approach outlined in the new research.
Like Dr Pankhania, he suggested that leaving out the elderly in early vaccination programmes would be poorly received.
“It would be seen as callous, almost as if you were consigning the older group to the scrap heap,” he said.
Amid starkly divergent views about who to focus on when mass vaccination starts, governments face not just logistical hurdles, but also the challenge of selecting the best strategy.