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Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold
[https://www.medrxiv.org/content/10.1101/2020.04.27.20081893v1]
"As the pandemic unfolds evidence will accumulate in support of low or high coefficients of variation, but soon it will be too late to impact public health strategies. We searched the literature for estimates of individual variation in propensity to acquire or transmit COVID-19 or other infectious diseases and overlaid the findings as vertical lines in Figure 3. Most CV estimates are comprised between 2 and 4, a range where naturally acquired immunity to SARS-CoV-2 may place populations over the herd immunity threshold once as few as 10-20% of its individuals are immune."
"The concept of herd immunity is most commonly used in the design of vaccination programmes. Defining the percentage of the population that must be immune to cause infection incidences to decline, herd immunity thresholds constitute convenient targets for vaccination coverage. (...) A crucial caveat in exporting these calculations to immunization by natural infection, is that natural infection does not occur at random. Individuals who are more susceptible or more exposed are more prone to be infected and become immune earlier, which lowers the threshold. The herd immunity threshold declines sharply when coefficients of variation increase from 0 to 2 and remains below 20% for more variable populations."
"Based on the spectrum of current knowledge, a high level of pragmatism may be required in policy responses to serological surveys. On the one hand, if CV is very low, the most stringent control measures would need to be continued for suppression of the epidemic. The other side of that coin is a scenario where keeping only the mildest control measures (protecting the elderly to reduce mortality rates) is optimal. It would therefore be imperative to conduct longitudinal serological studies in representative samples of the population, as control measures are relaxed."