Last week in parliament, Matt Hancock explained to the house why, “on the substance”, the central claim of the Great Barrington Declaration was “emphatically not true”.
“Many diseases never reach herd immunity – including measles, malaria, AIDS and flu…,” he said. “Herd immunity is a flawed goal – even if we could get to it, which we can’t.”
Let’s have a look at the diseases he mentions. Measles, if it arrives on ‘virgin soil’, can devastate a population. In Tahiti and Moorea and the South-east and North-west Marquesas, between 20% and 70% of the population was lost to the first epidemic. Natural infection with measles provides lifelong immunity, and we now have a vaccine that provides similar solid, durable protection. We have not been able to eliminate the disease, but those who rather selfishly choose not to vaccinate their children are only able make that choice because the risks of infection are kept low by those who are immune – currently, a combination of those, like me, who caught it and recovered and many others for whom it is vaccine induced. The vaccine does not work in babies, which is why you have to wait till they are a year old before they get it. We can do this because herd immunity keeps the risk of infection down, so they are unlikely to be infected in their first year of life. Without this herd protection, many under-ones would die (as they regularly do in sub-Saharan Africa) despite a vaccine being available.
Malaria (a primary focus of my research) is caused by a parasite which carries at least 60 different outfits with which to disguise itself from immune attack. We, nonetheless, typically acquire sufficient immunity upon first infection so as not to suffer severe disease and death upon further attacks. This is a feature it probably has in common with covid-19. But that is where the similarity ends. It has been difficult to make a malaria vaccine that covers all of its diversity, but fortunately that should not be a problem for covid-19.
AIDS is caused by the human immunodeficiency virus, which also has an extraordinary capacity to change outfits during the course of infection. This, and other mechanisms of subverting the immune response, allow it to persist indefinitely in an infected person. Even so, rates of infection will slow down as the virus runs out of people to infect.
Herd immunity is a continuous variable which increases as people become immune and decreases as they lose immunity or die
Fortunately, covid-19 does not possess such an array of immune evasion mechanisms that could seriously compromise any possibility of vaccine development in the next few years. Like HIV, influenza also has the ability to change its outfit, and periodically a new strain emerges requiring a new vaccine. It is because sufficient immunity accumulates in the population that a radical change of outfit becomes worthwhile. What Mr Hancock means when he says we do not reach herd immunity to flu is that it finds ways around it; it is unlikely, given the nature of coronaviruses that the SARS Cov-2 virus would be capable of doing the same thing.
