After eight months of chaos and more than 1 million deaths worldwide, we might soon have a vaccine for the novel coronavirus. With as many as 150 vaccines in development — nearly 40 in late-stage clinical trials — it could come by the end of this year, although most experts see 2021 as more likely.
Getting a vaccine is one thing, but distributing it is another. There won’t be enough doses for everyone, at least early on, raising the delicate matter of who should get the vaccine first. Even if we agree on the goal — say, saving as many lives as possible — there may be no obvious answer. Vaccinating the most vulnerable probably makes sense for this virus, as nearly all fatalities occur in the 65-and-over age group.
Yet epidemics work in counterintuitive ways, with subtle details of how the virus or vaccine affects different people carrying huge weight. Modeling studies suggest that lots of good could also come from focusing resources on who is most likely to be spreading the virus — in this instance, younger people and children. Before we get a vaccine, we need far more research to investigate how it might be best deployed.
Experts have already developed draft guidelines for the deployment of early vaccines. An advisory group of the World Health Organization has proposed focusing on the protection of older and more vulnerable people, essential health-care workers and groups in dense urban environments. In a report released last week, the U.S. National Academies of Sciences, Engineering and Medicine offered a framework for the equitable distribution of any vaccine; it emphasizes much the same, adding to the WHO group’s list of people working in essential industries such as food and public transport.
These guidelines mostly focus on using the vaccine to protect individuals as targets of the virus, as opposed to using vaccination to slow viral transmission. But this also helps: The less prevalent the virus, the less that vulnerable people need protecting. In the right conditions, in fact, some epidemiological research suggests that vaccinating the most vulnerable may not be the right thing to do.
A modeling study published in 2009 looked at vaccination strategies for influenza and concluded that the optimal policy — considering various measures including deaths and economic costs — depended on a number of things. If the vaccine was only somewhat effective, immunizing only around 50 percent of those vaccinated, then focusing on the most vulnerable was the best strategy. With low vaccine efficacy, even vaccinating most of the population wouldn’t prevent continued viral spreading, and so the vulnerable needed direct protection.
In contrast, if a vaccine was very effective, immunizing a higher proportion of those vaccinated, then the best strategy would target vaccinations not at the most vulnerable but at those most likely to spread the virus: healthy younger people, including children, who tend to be more socially active. High vaccine effectiveness makes it possible to greatly curtail viral spread, thereby making the most vulnerable — even unprotected — far less likely to be exposed to the virus.
It also mattered within the study how many doses of the vaccine were available. With few doses available, it is best to vaccinate the most vulnerable. If there were enough doses to vaccinate a decent fraction of the population, then targeting the spreaders was a better idea, as the achieved immunity level could eliminate viral spreading, although the required fraction depended on how easily the virus can be transmitted.
This study was for influenza, not the novel coronavirus, so its conclusions can only be suggestive. Even so, it made comparisons using data from both the 1918 and 1957 influenza epidemics, the latter of which was more coronavirus-like, with fatalities strongly skewed toward older people. In that case, the researchers found that the best strategy targeted the most vulnerable yet also allocated vaccines to younger people and children to reduce viral transmission.
As far as I can tell, no one has published a similar study for the current coronavirus. Yet other studies also suggest that stopping viral transmission should probably be part of any vaccine distribution. For example, physicists in Germany recently argued, using a statistical model, that focusing vaccination efforts in current hot spots — specific cities, for example — could be hugely beneficial. If good testing data were available and used to target vaccinations in places where the virus is spreading rapidly, they estimate that the overall number of deaths in an epidemic outbreak could be cut roughly in half. This is only a mathematical model, of course. But it makes a very similar point to the earlier study on influenza: Stopping transmission matters a lot, whether it focuses vaccinations on physical locations of enhanced spreading or on particular social groups.
Protection is only one part of vaccination. Much will depend on the nature of the early vaccines we have. But it seems likely that the best ways to stop the virus will need to go beyond simply targeting the most vulnerable. We’ll need more sophisticated methods to pinpoint those places and groups of people that the virus is finding most useful for multiplying its numbers.
Mark Buchanan, a physicist and science writer, is the author of the book “Forecast: What Physics, Meteorology and the Natural Sciences Can Teach Us About Economics.
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