Prof Francois Balloux is director of the University College London Genetics Institute. His work focuses on the reconstruction of disease outbreaks and epidemics. With his colleague Dr Lucy van Dorp, he led the first large-scale sequencing project of the Sars-CoV2 genome. During the pandemic, he has become a prominent scientist on Twitter, where he describes himself as a “militant corona centrist”. Would you say a new variant of concern is still the major threat to our way out of this pandemic? We haven’t had one in a while. The four variants of concern all emerged in the second half of 2020, and it’s important to keep in mind that viruses evolve all the time at a fairly regular pace. However, with Alpha, something unexpected happened: there was a sudden accumulation of mutations. One reasonable hypothesis is that Alpha emerged from an infection of an immunocompromised person or someone who was infected for a very long period. The other three variants of concern (Beta, Gamma, Delta) emerged through gradual accumulation of mutations. So it’s not always predictable? It’s extraordinarily difficult to predict a shock like the Alpha variant. What is easy to predict is that mutations will continue to appear and the virus will progressively drift, with the vaccines becoming less effective over time. There was a suggestion in a Sage paper that a very lethal variant could emerge, while other scientists suggest that the virus has reached its “maximum fit”, that if it evolves further it will lose the ability to coexist with its human hosts. It’s important to balance the scariness of predictions with their likelihood. The likelihood of a lineage emerging that is 50 times more lethal is extraordinarily implausible. I say that because we have 200 respiratory viruses in circulation and most of us get infected on a regular basis. We’ve never seen that kind of sudden change in mortality. I’m not saying it’s impossible, but you may have a better chance of winning the lottery jackpot many times over. Where does the emergence of the alpha and delta variants sit on your jackpot-winning measure? That’s such a difficult question. It is somewhat comparable to asking what the chances of someone winning the jackpot are, without having any idea about how many numbers there are on the lottery ticket. The Alpha and Delta variants emerged, and they obviously were winning combinations of mutations for the virus – though we also know that no other comparably transmissible viral lineage has emerged so far, despite millions of infections and a constant influx of mutations. Another concern is a mutation that enables the virus to “escape” the vaccines… Over two million viral samples have been sequenced, and we’ve probably already seen all the mutations that are technically possible. From our observations, we know that vaccine escape will not appear after one or two mutations – it will require an accumulation giving rise to the right combination. We will not go from one day everyone being protected to everyone being vulnerable the next. We will have time to update the vaccines. Also, while a vaccine-escape variant would indeed be able to infect vaccinated people far more easily, it would not nullify the protection against severe disease and death provided by the vaccine and prior infection. Where do you stand on vaccinating teenagers? This is the mother of all questions. There are people who are very passionate on both sides. Given the data available – and not many teenagers have been immunised – I think the JCVI has probably been right to err on the side of caution by first recommending the vaccine for healthy over-18s, and as more data has become available, to healthy over-16s. There is an issue with heart inflammation in younger males administered mRNA vaccines. One possible solution could be to give teenagers only one dose – most of the side-effects have been registered after the second dose. However, a single-dose regime hasn’t been trialled or approved yet. You have stated that a “non-trivial” number of long-Covid cases are psychosomatic. We know that infections such as Covid lead to post-viral syndromes. At the risk of being insensitive, I would be surprised if there wasn’t a link between disease severity and the severity of follow-up symptoms. Like tuberculosis or influenza, people who have a severe case should expect to take a long time to recover fully. And sometimes recovery is never complete. The situation is more complicated with a mild infection. Post-viral symptoms can happen but it seems relatively implausible to me that this would happen very frequently. In all likelihood, some cases are psychosomatic – though this doesn’t make the suffering less real for those affected or reduce the cost to society. All disease is real, irrespective of its root cause. There is a mental component to health and disease. Just the fear of something bad happening to us can make us feel unwell. A remarkable example of this process can be seen in the way over 30% of the people who were enrolled in the control arm of the Pfizer vaccine trial reported headaches and fatigue, despite not being injected with a vaccine. You had a bad bout of Covid – does this inform your view? I try to discount my own experience when I think about public health issues. That said, I would probably be included as having long Covid because six months later I haven’t fully recovered my sense of smell or taste. I think we need a meaningful definition that captures whether you have regained full fitness. I would like to stress: if you have a serious infection, do not necessarily expect to be back to full fitness in three months. Your Twitter bio states you are a “militant corona centrist”. What do you mean by that? From the start, the pandemic has been polarising. Some people thought we should “let it rip” or “take it on the chin” and others thought we should fight to eliminate it everywhere. These camps have fought for 16 months and it’s pretty toxic. Both are pretty extreme and unrealistic. I always thought very careful mitigation could keep the pandemic under control until vaccines arrived. A few countries have come close, such as Singapore, Norway and Denmark. The scientist is supposed to revise their conclusions as the evidence evolves. Do you feel that scientists on both sides of the debate have been holding on to their views in spite of the evidence? It’s self-serving to say “they don’t change their mind, I do” – despite the fact that I didn’t. Our brains work in a Bayesian way – we have priors that influence how we regard new information. As a scientist, it is very important not to have overwhelmingly strong priors – you need to be open to surprise and to let your priors be updated by new data. It’s important to engage with new evidence. Being dogmatic is problematic. This issue is amplified when, as now, scientists are talking directly to the public… Before the pandemic, scientists were rarely asked anything, or we were listened to in a polite, slightly bored way. But now people are clinging to the words of scientists, which can make it more difficult for them to change their mind. Few scientists have changed their views on Covid but when they do it’s often not well received – there’s an element of groupthink and for more media-savvy scientists, an expectation from their adoring crowd that they’re not meant to do that. Neil Ferguson has been criticised for his predictions of 100,000 cases a day after the easing of restrictions on 19 July. I know Neil, he was my boss for five years. His predictions were quite pessimistic but he tends to be pessimistic – which isn’t a criticism. If you are in a position of authority giving advice to the government you really want to err on the side of caution. Can you explain what you mean by “scientific populism”? As the pandemic has advanced the mood of the public has become darker and more fearful and this has created a market for gloom and doom. It’s as bad as the effects of the super-optimism at the beginning – stay at home for two weeks, it’s a mild disease or wear a mask and it will be gone. So I kind of captured the market for corona centrism – not to be systematically optimistic or pessimistic and to make it clear there are major uncertainties. And this is empowering, because understanding things is. You’ve often stated that the pandemic will be over by mid to late 2021. Do you stand by this? Depends on how you quantify it. I would say the pandemic is over when Covid-19 doesn’t cause significantly more mortality than other respiratory viruses in circulation. This will happen first in places such as the UK that have been privileged to get vaccine coverage – I expect at the latest early next year.