Coronavirus: Natural herd immunity to COVID-19 'impractical', suppression much better strategy - study

Achieving herd immunity against COVID-19 whilst avoiding mass casualties by adopting a mitigation strategy against the virus is virtually impossible, scientists say.

Instead, modelling shows suppression - the strategy adopted by New Zealand, China, Italy, Spain and South Korea in response to the initial outbreak - is the best approach, according to a new study.

COVID-19 has been linked to more than 981,000 deaths worldwide since January, with 32 million confirmed infected. After a peak in April, infections began to slow - some countries managing to completely flatten the curve by going into lockdown.

But lockdowns come at a huge financial risk - while short, sharp periods of restrictions that eliminate the virus allow economies to get back to normal quickly, they are incredibly disruptive in the short-term; and the long-term economic effects of going in and out lockdown, seeking to suppress the virus' spread whenever it appears, remain to be seen. 

Some countries - most notably Sweden - early on went with a strategy to achieve herd immunity, known as mitigation. Restrictions were light and largely voluntary, aiming to let the virus spread slowly to avoid overwhelming hospitals and hurting the economy. 

Herd immunity is when so many people are immune to a disease, outbreaks are impossible with a pathogen's access to potential new hosts in short supply.  

"Establishing herd immunity in a population by allowing the epidemic to spread, while mitigating the negative health impacts of COVID-19, presents a tantalising resolution to the crisis," University of Georgia infectious disease experts Tobias S Brett and Pejman Rohani wrote in their study, published in journal Proceedings of the National Academy of Sciences.

"Our study simulating SARS-CoV-2 spread in the United Kingdom finds that achieving herd immunity without overwhelming hospital capacity leaves little room for error."

So little room, in fact, they deemed it "impractical".

Firstly, to keep hospital beds from running out, the virus' reproduction number - the number of people the average infected person will go on to infect - will have to be forced so low (through distancing and other restrictions) that suppression, a vastly preferable strategy, becomes possible with "only a comparatively small further increase in control measures".

Secondly, if officials decide to doggedly pursue herd immunity it could take years of adjusting and tweaking social distancing rules and other restrictions to keep the reproduction number in the right zone - not too high, not too low. 

The scientists did come up with a formula to achieve this, but it required "knowledge of unobserved epidemiological determinants, namely the remaining susceptible population, fraction exposed, and hospitalisation probability". 

In other words, it worked in their computer simulation with perfect information - but in the real world, no one has perfect information. With asymptomatic transmission, no one really knows how many people have been infected already, and as a new disease, just why some people fall sick and others don't is still being investigated. 

It's also not known yet if recovering from the virus even confers immunity  - there have been reports of people contracting the virus a second time. It's also not known what percentage of the population need to be immune to achieve herd immunity - it's different for each virus.

"If immunity is not perfect, and there is a moderate to high chance of reinfection, then prospects for achieving herd immunity via natural infection are slim," the scientists said.

Some countries which managed to suppress massive outbreaks earlier this year are now experiencing second waves - Spain and Italy in particular. Neither are yet to enact the same kind of stringent lockdowns they did in April. The UK never completely stopped transmission, but reduced it by about 90 percent between April and July - it's now back up to where it was at the height of the first wave.

"Given the potentially long wait until a vaccine is available, the UK government appears to have considered following Sweden's example and attempt to achieve herd immunity in the country."

Despite the ever-increasing speed of the virus' spread, deaths thankfully haven't quite kept up - a number of reasons have been given for this, including better understanding of how to treat the disease and a higher proportion of young people being infected this time around. 

After an initial wave of deaths larger than its locked-down neighbours, Sweden managed to get its daily new case numbers down - but there are growing signs of a second wave there too. And despite their widely publicised lack of lockdown, according to Oxford University's Government Response Stringency Index - which measures how tough each country's response has been to the virus - New Zealand currently has fewer restrictions than Sweden. 

Comparing the lockdowns between Sweden, the UK, Italy, Spain and New Zealand.
Comparing the lockdowns between Sweden, the UK, Italy, Spain and New Zealand. Photo credit: Our World In Data

Sweden has had 5876 deaths compared to New Zealand's 25 - about 115 times more, when adjusted for population. 

"Our modeling confirms that suppression is possible with plausible levels of social distancing and self-isolation, consistent with experience in multiple countries," the study concluded. 

"Our research does not, however, support attempting to mitigate COVID-19 with the aim of building herd immunity... 

"Various governments have entertained the idea of achieving herd immunity through natural infection as a means of ending the long-term threat of COVID-19. This has provoked alarm in sections of the public health community. Our work confirms that this alarm is well-founded."

A number of vaccines are in development, a few of which have reached the all-important third phase of their trial - when they're injected into tens of thousands of volunteers to see if it protects them against the disease.