“Four EU countries are close to natural herd immunity”

October 5, 2020 0 By FM

Models suggest that at least 60% of individuals in a given population need to be immunised to halt further transmission of the virus in that community. The 60% plus threshold for herd immunity is the minimum coverage needed to interrupt transmission by using random vaccination. But a recent preprint postulates that natural herd immunity for SARS-CoV-2 can be reached at a far lower threshold – of around 10 to 20% — through the immunity induced by an infection which is naturally selective. Individuals who are more susceptible to or exposed to the virus tend to be infected and become immune earlier. These variations in susceptibility or exposure lower the herd immunity threshold compared to vaccination. Hence some populations may be close to achieving herd immunity despite being under more or less strict social distancing measures, according to the study titled: Herd immunity thresholds for SARS-CoV-2 estimated from unfolding epidemics, which appeared in medRxiv.

Gabriela Gomes, Professor of Mathematics and Statistics at the University of Strathclyde, Scotland and the lead author, tells FM that the findings of the study have profound consequences for the governance of the current pandemic. Edited excerpts:

 Could you briefly explain how exactly you arrived at the conclusion that the natural herd immunity threshold for SARS-CoV-2 could be as low as 20%?

 The reason why herd immunity thresholds are lower when immunity is induced by natural infection than by vaccination is individual variation in susceptibility or exposure. In a heterogeneous population, individuals are not infected at random. Specifically, those who are more susceptible or exposed tend to be infected and become immune earlier. Since an individual’s contribution to population immunity is weighted by his susceptibility and exposure, the more variable these characteristics [within the population] the lower the proportion needed to achieve a given level of immunity, such as the herd immunity threshold. We applied models that account for complete variation in susceptibility or exposure to a series of cases in four European countries and estimated herd immunity thresholds around 10-20%.

 Are lower immunity thresholds for natural infection common, or is it specific to SARS-CoV-2?

 The variation and selection that explains the relatively low herd immunity thresholds when immunity is induced by natural infection is general. The idea has not been very prominent until now, probably because we only see the phenomenon in action when we have an emerging infection that spreads widely, which fortunately does not happen often.

 A recent serosurvey conducted in Spain came out with the finding that only 5% of the population had antibodies against SARS-CoV-2 despite the higher rate of infection in this European country. According to your estimates, in what ways can the introduction of a vaccine against SARS-CoV-2 influence the progress towards herd immunity induced by infection? 

 I think all our 4 study countries (Belgium, England, Portugal, Spain) are close to herd immunity. Our latest paper includes a comparison of our model outputs with reported seroprevalences in Spain. There is substantial geographical variation in seroprevalences even within a country. In Spain some regions have more than 10% population with antibodies and others much less. Because of heterogeneity in geographical spread, achieving herd immunity is a gradual process. A patchwork forms combining regions that have been highly affected and are protected and regions that remain vulnerable to outbreaks. We have to analyse each setting beyond that single number that describes the national herd immunity threshold.

 In what ways, the introduction of a vaccine against SARS-CoV-2 can impact the pace of achieving herd immunity induced by infection? 

 If a vaccine is introduced in populations that have not yet crossed their herd immunity thresholds, herd immunity will eventually be achieved by the combined action of natural infection and vaccination. Because vaccination programmes do not necessarily target those individuals who are more susceptible and exposed to infection, herd immunity thresholds are expected to be higher through this route. I suppose recommendations will be released in due time for how to target SARS-CoV-2 vaccines, probably to people at higher risk of severe outcomes of infection, and possibly potential super spreaders.

 Reports of reinfection by SARS-CoV-2 are already coming up. How far can the reinfection potential, that too within a short span after recovery, affect the natural herd immunity threshold? 

 Based on their rarity so far, reinfections are not expected to have much impact on the herd immunity thresholds that will take the pandemic to a halt. Reinfections will have a more important role in the post-pandemic phase when the virus becomes seasonal.