COVID-19, The Next Flu Season, And The Temporary Immunity Hypothesis

Credit Spencer J. Fox

#15,297

Much like the original SARS epidemic in 2002-2003 - SARS-CoV-2 began to spread in China in the fall - only to emerge on the world stage in February and March.  This is a pattern we've also seen with influenza pandemics

The 2009 H1N1 pandemic virus was first detected in late April, while the 1957 H2N2 pandemic first showed up in the United States early in the summer. 

Two and a half years ago, in PLoS Comp. Bio.: Spring & Early Summer Most Likely Time For A Pandemic, we looked at a fascinating study that found that of the 6 influenza pandemics in the Northern Hemisphere since 1889, all of them first emerged or saw their first significant wave in the spring or early summer.

Using a computer model, researchers at the University of Texas at Austin found evidence of a narrow window of opportunity for pandemic emergence. The authors offered two possible factors behind this trend;

1. Respiratory viruses spread best under winter environmental and social conditions
2. People who are infected by one respiratory virus can develop temporary immune protection against other respiratory viruses

Given the seasonality of most respiratory viruses, the first option may seem obvious. The second, based on what has been dubbed the `Temporary Immunity Hypothesis', is perhaps less intuitive. 

While not exactly proven, this idea has been around for some time.

In the fall of 2009, at the height of the H1N1 pandemic, there were some regions of the world that reported a large spike in rhinoviruses, but relatively few pandemic cases.  I referred my readers to a piece on Effect Measure on this phenomenon  (alas, no longer online) in Referral: EM on The common cold and influenza. 

The article that sparked that discussion, written by Debra McKenzie of New Scientist, is still available (but behind a paywall).

Common cold may hold off swine flu

A year later an article appeared in the Eurosurveillance Journal (see Eurosurveillance: The Temporary Immunity Hypothesis) that suggested that contracting seasonal flu (as opposed to being vaccinated against it) temporarily ramped up the body’s immune system against other viruses – and that this protective effect could last months.

While flu is the most obvious catalyst, there is some evidence that other respiratory viruses may have the same effect.  Getting a nasty rhinovirus appears to offer some temporary protection against influenza, for instance. 

Dr. Ian Mackay discussed a similar hypothesis in his blog back in 2014, in Influenza in Queensland, Australia: 1-Jan (Week 1) to 8-June (Week 23), where he suggested that the immune response to the early spread of one respiratory virus might dampen the spread of a second virus - perhaps for months - what he dubbed a `shields up' effect. 

Given that much of the world has been either on lockdown, or has been practicing social distancing and better hand hygiene for the past 2 or 3 months, we've seen a dramatic reduction not only COVID-19 infections, but also on the transmission of influenza, and other diseases. 

Nowhere is that trend more apparent than in the UK's precipitous drop in scarlet fever cases this spring (see chart below).

Social distancing has not only worked for COVID-19, it appears to have reduced our overall infection rate on a variety of fronts.  Arguably, a very good side effect. 

But if (and its still an `if') the temporary immunity hypothesis holds true, and we go into this fall with `our shields down', we could be setting ourselves up for a very nasty flu season on top of whatever else COVID-19 decides to do. 

Last winter, we saw primarily H1N1 and influenza B, which means that we may be ripe for a return of H3N2, which tends to produce more severe flu seasons. Aside from the morbidity and mortality that would produce, it will also severely complicate COVID-19 surveillance, contact tracing, and (hopefully) suppression.   

While all of this is admittedly speculative, I would submit that this fall - perhaps more than anytime in recent memory - getting the flu shot should be high on your to do list.

No, it won't protect against COVID-19, and unless we get very lucky, it likely won't provide more than 50%-70% protection against influenza.  But even if it doesn't prevent infection, it may help reduce the severity of illness, and that may reduce the number of heart attacks and strokes next winter.

This summer, even as states gradually reopen and people begin to emerge from their stay-at-home lifestyle, we are likely to continue with an elevated level of social distancing and hand hygiene. People may be inclined to participate in fewer `healthy' summer activities - like outdoor exercise and going to the beach.   

Mask wearing in public, at least in enclosed spaces, is likely to continue. 

Unless we see some kind of summer virus (e.g. EV-D68, hCoV) sweep the country, by November  many of us will have gone 6 or 8 months without having had a respiratory virus.  And that - at least, in theory - suggests our viral shields may be lower than normal going into the fall. 

Exactly what happens this fall when our normal flu season, COVID-19, and our immune systems collide is still anyone's guess. We will hopefully know a great deal more about COVID-19, its interactions with influenza season, and the long-term effects of social distancing a couple of years from now.

Until then, we need to use the tools we have to mitigate its effects.  

Which is why this year, in addition to social distancing, hand hygiene, and wearing face covers in public, the seasonal flu shot will be part of my pandemic toolkit.