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Virulence evolution and herd immunity

The billionaire hedge fund manager Ricky Sandler says we should disregard public health professionals and encourage people to get infected with COVID-19 so that the population can have “so called” herd immunity.

He and others argue we should throw caution – and our face masks – to the wind and promote infections in younger people. Sandler says that he “envisions young and healthy people attending ‘virus relief concerts’ in hopes of getting the virus.” I can guarantee that Sandler hasn’t given any thought to the evolution of virulence, let alone gathered any knowledge on public health basics.

Paul Ewald, the author of the book “The Evolution of Infectious Disease” pointed out that transmission characteristics can shape the virulence of parasites and pathogens. He explained why microbes like Vibrio cholerae have evolved to be deadly pathogens, untamed despite a long coevolutionary history with humans. Hypervirulent cholera does not rely on its host to be alive to transmit to the next susceptible host. Cholera spreads efficiently in water used for drinking, even after a victim has died of the disease. Diseases that spread in the water, like Cholera, or persist for a long time in the environment, like Anthrax, pay little cost when they kill their host, so they remain highly virulent over evolutionary time.  

Being spread by vectors is also linked with higher virulence. Having an insect vector can create the conditions under which high virulence is selected for. An incapacitated host, after all, is one that does not swat a visiting mosquito away.  Falciparum malaria, spread by mosquitos, is a persistent killer of humans, showing no sign of evolving to become more benign over time.

 In similar fashion, medical workers interacting with incapacitated hosts create a selective environment that permits highly virulent pathogens to spread successfully. This is one reason why hospital acquired infections are generally worse for the victim than community acquired infections. Doctors and nurses inadvertently select for high virulence when we fail to wash our hands, or these days, fail to wear a mask. Modern health care, with acute care hospitals and EMS systems tend to cluster sick individuals in hospitals, where medical attendants then transmit disease between incapacitated infected hosts. These events have been documented during the COVID-19 pandemic, contributing to super-spreader events in Wuhan China and in Lombardy, Italy.

As we have discussed, tradeoffs are important in shaping the evolution of disease traits.  For the evolution of virulence, a tradeoff exists between virulence and the duration of infectivity. A highly virulent pathogen might sometimes kill its hosts before it has a chance to transmit to a new host. Directly transmitted infections like Marburg virus and Ebola virus may be too virulent for their own good, with epidemics that burn out before they can become deeply established in a human population.

COVID-19 is spread mostly by direct contact, when it is not being transmitted in hospitals. So, should the coronavirus evolve to be less virulent over time? One reason for its severity might be the fact that it recently switched hosts. The “old host,” bats, have a different immune system, and coronaviruses generally cause mild disease in their horseshoe bat hosts. Host switching may cause a mismatch between virulence a pathogen has in its old versus its new host. With a new host and a novel pathogen, it is likely that the virulence characteristics of a pathogen (and the immune responses of a host) might be non-optimal. Over time, we might expect that both of these characteristics – host immunity and pathogen virulence – will change depending on disease ecology, and public health measures.  

Most pathogens that are transmitted with direct interactions between people to evolve a lower level of virulence, because viral fitness relies on individuals engaging in face-to-face contact for successful transmission. COVID-19 is a strange case because even though average virulence might be more severe than for cold viruses or seasonal influenza, many people have little or no symptoms.  If we are stuck with COVID-19 for the long term, do you think it will evolve to be more or less virulent over time?

Public health measures, like hand washing and mask wearing, are also expected to have an effect on the evolution of virulence, because decreased transmission opportunities should favor more indolent, less harmful variants of a pathogen.

Suppose then, that Ricky Sandler got his way, with COVID-19 concerts and parties. If we did this, what would it mean for the evolution of virulence?

Update: I posed the question above to my students. Here is my answer.

Having lots of COVID parties that make transmission easier would encourage evolution towards increased virulence.  Anything that reduces the friction to transmission also makes it easier for a super virulent virus to spread. So, if the virus has many quick opportunities for transmission then it does not pay a fitness cost for increased virulence. Even if a virus kills you at day 5, if it is able to transmit to several people on days 1-4, it will do very well. This is especially so if it makes even more copies of itself during the short infectious period.

Imagine this scenario: Let’s say we all wear masks and gloves and limit our interactions so that it takes 10 days between infection and transmission to the next susceptible host. The 5 day killer virus will go extinct.  Another strain, one that simultaneously reduces its replication rate and has a longer duration of infection, will have higher fitness than the 5 day killer variant. If on the other hand, transmission happens in 2 days, the super virulent strain will actually do better, because during that 2 day period, the virus will make more copies of himself than the less virulent strain. Those variants would do better. So our policies could have an impact on the virulence characteristics of COVID-19. If the tradeoff model is correct, then social distancing and mask wearing will favor decreased virulence. COVID parties would be expected to the opposite.

There are so many other problems with the Ricky Sandler proposal. One being the idea that everybody gets to be social and mask-free, except for the vulnerable, who need to be “protected.”  Besides the wrong-headed notion that a huge chunk of the population is considered useless and disposable, vulnerable people can’t just be quarantined on an island to fend for themselves. They necessarily interact with younger caregivers and family members, who would put vulnerable people at even greater risk! And any attempt to separate the “vulnerables” would necessarily concentrate them together, which increases the risk of outbreaks, and provides greater selective pressure for increased virulence.

Categories: Uncategorized

Joe Alcock

Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine

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