COVID-19 herd immunity threshold far higher than previously thought, say researchers

By Dr. Ananya Mandal, MDDec 4 2020

Researchers in the US have re-analyzed the data on herd immunity from spring 2020 from Europe. Their study shows that the herd immunity threshold is much higher than earlier speculated. The team – from the University of Texas at Austin and The Santa Fe Institute, New Mexico – have released at the preprint server medRxiv*.

Study: The COVID-19 herd immunity threshold is not low: A re-analysis of European data from spring of 2020. Shutterstock / tolgaildun

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

The COVID-19 pandemic was detected late in 2019, in Wuhan, China, and by the 11^th of March 2020, it was declared a pandemic by the World Health Organization (WHO). It is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has infected over 65.67 million people around the world and taken the lives of over 1.51 million people. It has been seen that those with the infection may have developed an immunity at least for a few months (as is known from current research).

Since the start of the pandemic, studies have tried to understand the population's immunity against the infection and the development of herd immunity, which would protect the vulnerable population by breaking down transmission chains.

Herd immunity and herd immunity threshold (HIT)

When a highly infectious disease affects a community and a large portion of a community (the herd) gets infected and becomes immune to the infection; the chances of spreading the infection between persons reduces. This results in the whole community being protected against the infection – not only those who are immune.

For the whole population to be protected, a large portion of the population needs to be immune to the infection – and this is called the threshold proportion. If the threshold is high, more people need to be immune to protect those who are not. Vice versa occurs when the threshold is low. When the proportion of the immune population is greater than the threshold, the infection's spread declines. This is known as the herd immunity threshold (HIT). Usually, highly infectious diseases have a high threshold. Measles, being highly contagious, for example, requires a 94 percent threshold to break the chain of transmission.

HIT – what is known

Early during the pandemic, a study by Aguas et al. hypothesized that the COVID-19 herd immunity thresholds (HITs) were low – at around 6 to 21 percent. The team of researchers on this study speculated that these numbers are "inconsistent with other model-derived estimates," and the seroprevalence studies from regions where the incidence of the infection has been high.

The Great Barrington Declaration (GBD) has recently called for relaxing the public health interventions such as physical distancing and prevention of gatherings for young, healthy individuals owing to herd immunity. This has been based in part on unpublished research that says the HIT is low at 10 to 20 percent. This study was conducted to see if these assumptions are true.

Study design

Seroprevalence studies are population tests for antibodies against the infection. This study reviewed the methods used by Aguas et al., and they assessed the level of mitigation efforts to prevent community transmission of the infection so that the COVID-19 HIT estimates could be moved lower. The team obtained the code used by the earlier study from the open-access GIT Hub.

For this study, the team modified the earlier model and analyzed the mitigation curves. Two forms of mitigation curves were studied – timing and magnitude of transmission reduction via non-pharmacological interventions. They also tried to derive alternate plausible mitigation curves from the "COVID-19 Government Response Tracker response index data". With the use of their numbers and models, the team tried to predict the COVID-19 mortality on the estimated HIT. They also attempted to see if their model was unable to "simultaneously estimate the impact of community mitigation and population heterogeneity." Their codes can be accessed here.

Findings and implications

A re-evaluation of the findings and numbers available, using the new mathematical models, shows that COVID-19 HIT estimates are as high as 60 to 80 percent. This shows that a high number of the population needs to be immune to the infection either by getting infected (active immunity) or by vaccination to protect the remaining non-immune population. This finding has significant implications on present policies that propose relaxing the mitigation measures, say the researchers.

The team writes, "Our re-estimates of the COVID-19 HIT corroborate strong signals in the data and compelling arguments that most of the globe remains far from herd immunity, and suggest that abandoning community mitigation efforts would jeopardize the welfare of communities and integrity of healthcare systems."

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.