Andrew Pollard: Deciding what to vaccinate against

As former chair of UK's Joint Committee on Vaccination and Immunisation, the JCVI, Andrew Pollard knows all about the difficult trade-offs facing decision makers for immunisation programmes. Science, safety, and cost-effectiveness are the key criteria...

Chris – We saw this manifest during the pandemic – that with certain vaccinations, it’s a one-stop shop. You either get the disease or, ideally, you get vaccinated, and with one or two doses, you never get that thing for the rest of your life. On the other hand, we were updating Covid vaccines almost monthly, it felt, during the pandemic. So why is there that distinction? Some things, it’s a one-off – others, not?

Andrew – If we take something like coronavirus or influenza, we need to have regular vaccines if we want to completely prevent infection with them, because the viruses are changing all the time and we can never quite keep up with them. If, through mutation, the virus changes, the vaccines we had last year are no longer as effective. That doesn’t mean they’re not effective at all, but they’re less effective, because the virus, as it replicates, makes mistakes in creating new copies of itself.

Some of those new copies completely evade the immune responses we’ve already made. So that means you can get reinfected with coronavirus repeatedly, even if you’ve been vaccinated. The important thing, with Covid-19, is that the vaccines – even those from 2021 – still prevent death, but they’re no longer very good at preventing infection. Those infections can still be quite miserable. But now, if you encounter coronavirus, you don’t get the overwhelming disease that we saw filling intensive care units, because the vaccines still offer good protection against that severe endpoint.

With other viruses, they don’t change much at all, and survive by infecting unvaccinated people. Measles is a good example of that. The measles vaccine still works brilliantly today, just as it did when first used in 1960, because the measles virus has hardly changed in all that time. But if you allow a pool of unvaccinated people in a community – as happens in countries with poor vaccine coverage – then the virus can still spread and cause explosive outbreaks.
But once you’ve been vaccinated, you’re protected for life because the virus doesn’t really change.

Chris – Presumably, if you have enough people in a population who are vaccinated, that’s where this concept of herd immunity comes in – because there are so few people left who can catch the infection that the chances of an infected person running into a non-immune person are so slight, you just can’t get a chain of transmission.

Andrew – That’s correct. You don’t get a chain of transmission. But for that unimmunised individual, if they did encounter someone with measles, they could easily get infected – and it’s so infectious, it’s highly likely they would. The reason for maintaining very high levels of immunity in the population is to stop those chains of transmission, both because measles can be fatal for children and adults, and because there are people in our community who cannot be vaccinated – for example, those with compromised immune systems or undergoing cancer treatment. Those people really need everyone around them to build that shield of immunity and keep the virus out.

Chris – How do you decide what we’re going to vaccinate against? There must be umpteen things we could theoretically protect everyone from, but we choose a certain suite of diseases for routine paediatric vaccination, and let people catch others. How is that decision made?

Andrew – The starting point, historically, for deciding what vaccines to develop has been to focus on the most severe diseases – those that cause death. And most of the childhood diseases that are relevant in the UK and occur at any scale, we now have vaccines for. It doesn’t mean we can prevent every infectious death, but we’ve made incredible progress, especially in the last 60 years – and really since the early 20th century, when the first vaccines were used more widely.

That was the initial focus. But today, now that we've addressed the major killers like diphtheria and measles, the emphasis has shifted towards ensuring that any new intervention we add to the NHS is good value for taxpayers.

That’s determined by a cost-effectiveness metric set by the government’s Treasury. What that means is that we assess new vaccines on the same basis as any other NHS treatment – so we don’t end up funding vaccines for minor illnesses at the expense of, say, a new cancer drug that could offer much more benefit.

If we just take the health service perspective, you can do very detailed calculations across all the medicines and vaccines potentially available to the NHS, and prioritise only those that meet the threshold for being good value.

Chris – And is that the role of the JCVI in the UK, the Joint Committee on Vaccination and Immunisation, which you chair? Is it the committee's job to weigh those things up and make recommendations?

Andrew – It is, absolutely. The starting point is whether there’s a burden of disease. We have excellent data in the UK from the NHS, collated by the UK Health Security Agency, on a wide range of diseases affecting the population.

Next, is there a vaccine? Academic and commercial developers work on products that they hope to offer to the market. We then look at the data and assess whether the vaccine works, whether it’s safe, what impact it could have in the population, and – crucially – whether it meets the Treasury’s rules on cost-effectiveness.