Importance of untested infectious individuals for the suppression of COVID-19 epidemics

Francisco Perez-Reche; Norval Strachan

Irrespective of the effectiveness of the lockdown, our model predicts that epidemics will resurge after relatively extended lockdowns in Hubei, Italy and Spain. The same is likely to occur in other countries. In fact, resurgence of the disease is predicted even for much longer lockdowns. This is due to the fact that an early lockdown delays the spread but does not lead to herd immunity. Assuming that recovered individuals are immune to SARS-CoV-2, herd immunity is only achieved when a proportion 1 − 1/ℛ 0 of the susceptible population has been infected and died or recovered. Even if the number of untested individuals that may have recovered are taken into account, we estimate that, as of 09/04/2020, the the proportion of the susceptible population that has been exposed to the virus (i.e. the attack rate, see Methods) is 0.65% [90% CI: 0.46%-1.18%] for Germany, 0.5% [0.3%-1.0%] for Hubei, 4.6% [3.3%-7.2%] for Italy, 3.7% [2.0%-6.4%] for Spain and 4.4% [2.7%-6.9%] for the UK. These proportions are small compared to the 70-85% needed to ensure herd immunity for these epidemics. Our conclusion is in qualitative agreement with the results by Flaxman et al. 3 despite the fact that their estimates for the attack rate tend to be higher than ours. Our conclusions, however, disagree with Lourenço et al. 14 that predicted much higher attack rates that would be close to herd immunity. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.