Communities which have experienced widespread transmission of COVID-19 might be better protected against future outbreaks than those relying on vaccination efforts, a new study has found.
But the scientists behind the research - and a local Kiwi expert - say that doesn't mean we should let the virus spread, urging people to get vaccinated to protect themselves and others.
Scientists in the UK and Sweden - two countries which early on opted for suppression, rather than elimination of the virus - used computer simulations to see how future outbreaks would go in communities with widespread previous infection with COVID-19, compared to those with similar levels of immunity but acquired through vaccines.
Specifically, they looked at what preventative measures such as closures, distancing and mask mandates would be needed to stop an outbreak, and how strict they'd need to be.
"The main conclusion from our study is that disease-induced immunity reduces the risk for a large future outbreak substantially more than when the same immunity level is achieved from vaccination," they wrote in the study, published in journal Royal Society Open Science on Wednesday.
How it works
Local COVID-19 modelling expert Michael Plank of the University of Canterbury and Te Pūnaha Matatini told Newshub the model showed that "countries where a significant part of the population has been infected with the virus will be better protected against future epidemic waves than countries that are relying solely on vaccination".
"This is because people with lots of contacts who are more likely to be superspreaders are also more likely to have been infected in the past and therefore be immune to the virus."
New Zealand is hoping to rely almost completely on vaccination, with one of the lowest infection rates in the world - just 2507 confirmed cases in a population of 5 million. The US, for comparison, has likey had more than 65 million cases - about a fifth of the population - according to a recent analysis.
In the new study vaccination was assumed to be taken up more uniformly across the community, achieving the same level of immunity overall - but leaving gaps in social networks amongst communities that haven't experienced a COVID-19 outbreak, that would likely have been filled during an uncontrolled outbreak.
"The virus is really efficient at finding people who are highly social or have lots of contacts," said Dr Plank. "Once those people have got infected, they can't (for the most part) get infected again. So next time the virus comes it has a much harder job finding people who can catch and spread it.
"On the other hand vaccination doesn't specifically target these highly social people… They will get vaccinated eventually (if they choose to) but it might take longer."
While the new study might be true "mathematically", Dr Plank notes it's just a simulation.
"There are other factors that can have the opposite effect. For example, if the virus has previously spread disproportionately within some regions or tightly-connected groups, this can mean there are significant parts of the population than remain vulnerable until they can be vaccinated.
"The model also assumed that that infection provides the same level of immunity as vaccination does and ignored effects of waning immunity. In reality, people who were infected early in the pandemic may be losing some of their immunity by now.
"Vaccine immunity may also wane over time but it's likely we can address this with boosters. In addition, by prioritising older age groups for vaccination, we can target protection to those at highest risk of severe illness or death."
Why vaccination remains a better option
Letting the virus run through the population is a bad way to achieve herd immunity, if it's even possible. Previous research Dr Plank has been involved with suggested as many as 97 percent of us might need to be immune to stop the ultra-contagious Delta variant from spreading. The latest study assumed an infection fatality ratio of 0.5 percent - so getting to 97 percent immunity in New Zealand that way would be condemning 25,000 of us to death and leaving thousands more suffering long-term effects.
"This result is by no means an argument against vaccination or strong early preventive measures, both of which we strongly support," the scientists behind the new study wrote, urging readers to interpret the results "with caution".
"It is still vastly preferable to obtain immunity through vaccination because this prevents the major health impacts that occur when the virus spreads through a population," said Dr Plank.
The UK is experiencing a third major wave thanks to the Delta variant, but thanks to its fast vaccine rollout hasn't experienced a mortality rate anything like it did earlier this year, when the daily death toll was frequently well above 1000. The worst day in the current wave - July 27 - saw just 131 die, despite a similar number of confirmed infections.
Around two-thirds of Brits have had at least one jab of the AstraZeneca, Moderna, Janssen or Pfizer-BioNTech vaccines. In New Zealand, it's around 19 percent.
Sweden, which hasn't had any full lockdowns but has had ongoing lower-level restrictions, has had 14,615 confirmed COVID-19 deaths to date. The UK has had 129,172. New Zealand, which pivoted from a suppression strategy to elimination early on, has had 26.
"The question of how contact patterns and population immunity affect the spread of the virus is a complex one," said Dr Plank."As the authors acknowledge, this study looks at the theoretical effect of one particular mechanism, ignoring other forms of variability. So at present these findings should be viewed as a mathematical hypothesis rather than an established fact."
