COVID-19 Day 52: Herd Immunity Could Cost Less Than 240,000 Lives

C19_herdimmunity

Perhaps you’ve heard of ‘herd immunity’? The term is often used to describe a national strategy to mitigate COVID-19, where business and schools remain open, and the population living ‘normally’, while letting the disease run its course and burning out naturally. In epidemiology ‘herd immunity’ means something different, which we’ll discuss shortly.

Sweden is attempting to achieve herd immunity by isolating its vulnerable population and banning large public gatherings, but largely allowing the population and business to function as normal. The UK once considered this approach but abandoned it after an Imperial College epidemiological model predicted as many as 500,000 Britons would die if they let the virus run its course, or as few as 20,000 if they imposed a national shutdown. The US followed the UK’s strategy for the same reasons.

The term ‘herd immunity’ comes from the epidemiological concept that, you don’t need to vaccinate 100% of a population to effectively protect the whole ‘herd’ from a pathogen. All you need is enough of people to be immune from SARS2 so that the virus can’t find a new un-infected host before it’s destroyed by its current host’s immune system. The hard way to achieve immunity from COVID-19 getting infected and recovering, which exposes some to the risks of the virus such as organ damage or death. That’s why vaccines are much better, because people gain immunity without actually suffering the real illness.

How many Americans need to be immune to achieve herd immunity here? Johns Hopkins estimated that it would take 70% of the US population. Are we close to achieving it? How many Americans are immune now? That’s a little tricky but we can do some back-of-the-envelope calculations to figure that out.

After 50 days of nationwide shutdown, the official count stands at more than 1 million Americans infected by COVID-19. Some studies suggest anywhere from 20-80 times more may have been exposed and recovered. If we extrapolate the USC study findings and estimate that 50 million Americans have recovered from the virus and are now immune, that’s only 15% of the US population, which means over 85% is still vulnerable to infection. If herd immunity requires 70% of the population immune, that’s about 230 million people. If we assume 50 million Americans are already immune, that means we’re still 180 million short of herd immunity.

Great, but getting to the other 180 million immune is not without cost. About 60,000 Americans have died from COVID-19 making for a suspected case-fatality rate between 0.1%-0.3% which is much lower than the dire 3.5% case-fatality rate understood at the beginning of March. Taking the best-case estimate, a 0.1% case-death rate may seem small but when you multiply that by the 180 million Americans needed for herd immunity that means we’ll experience 180,000 more deaths to get there. Added to the 60,000 who have already died and we would see a death toll of 240,000 Americans. That’s 3-4 times more deaths than the seasonal flu. Only the 1918 Spanish Flu Pandemic killed more Americans (an estimated 675,000).

One of the key concerns about reopening the country relates to the 1918 Spanish Flu. That pandemic occurred in 3-4 different waves months apart. As populations got infected, like today, they isolated and local epidemics burned out, but months later the virus returned from other parts of the globe on ships carrying passengers, solders, and refugees from World War 1. Today’s air travel means even more of the globe is reachable in hours instead of days.

It’s believed that 10x as many died in the second wave of the Spanish Flu than the first wave. Why so many more? One theory is that the virus mutated to a more deadly strain after the first wave. Or perhaps, people were lulled into a false sense of security and didn’t continue safety and hygiene procedures after thinking, hey the first wave wasn’t that bad.

Public Health Officials are aware of this tragic history and are concerned that relaxed vigilance could create conditions for a more lethal 2nd or 3rd wave of COVID-19 as we ease lockdowns. People are already lowering their guard, unconcerned about catching infections as they protest in states capitols. Perhaps like the people of 1918, they too think they’ve survived the worst of pandemic and it wasn’t that bad?

Perhaps seeking herd immunity is the wrong goal? We suffer as many as 60,000 flu deaths each year but our goal was never been to achieve herd immunity for that. Our goal each flu season is to avoid catching it and we’ve never needed to close down our economy and hide at home to do that. In fact the CDC estimates only 5%-20% of the country catches the flu every year.

If we’re to avoid a deadlier 2nd wave that some predict, it will be largely dependant on rapid testing and the population’s responsible behavior. I know, that’s kind of scary right there. As we emerge from lockdown and go back to work, school, and a more ‘normal’ way of life, it’s important to remember that if the Johns Hopkins estimate is correct and if the USC estimate is correct, we’ve still a long way from achieving herd immunity. Maybe instead of seeking herd immunity, we should just seek to stay healthy.

We’ve heard of the motto ‘bend the curve’, now we need to learn a new one: ‘stay vigilant’.

 


SOURCES

https://www.theatlantic.com/technology/archive/2020/04/coronavirus-models-arent-supposed-be-right/609271/

https://www.jhsph.edu/covid-19/articles/achieving-herd-immunity-with-covid19.html

https://news.usc.edu/168987/antibody-testing-results-covid-19-infections-los-angeles-county/

https://www.cdc.gov/flu/pandemic-resources/1918-pandemic-h1n1.html

https://www.livescience.com/covid-19-pandemic-vs-swine-flu.html

https://www.cdc.gov/flu/about/keyfacts.htm

https://pressroom.usc.edu/preliminary-results-of-usc-la-county-covid-19-study-released/

https://www.usatoday.com/story/news/factcheck/2020/04/25/fact-check-total-deaths-each-spanish-flu-wave-unknown/3024648001/

https://www.cdc.gov/flu/about/keyfacts.htm

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