Note: I’ll be out of town on vacation next week, so Q&A will be paused until 6/22
Question: Could the declines in new cases and deaths in Europe be due to herd immunity?
Answer: Many European countries have seen steep decreases in new cases and deaths over the last several weeks (Figure 1 and Figure 2 from Our World in Data). Two studies published yesterday in The Lancet show that these declines are not because of herd immunity. Thus far, the scientific consensus is that no country has seen infection rates sufficient to prevent a second wave. For a refresher on herd immunity, see our Q&A of 5/1. Now on to the results of those two studies:
- Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Geneva, Switzerland (SEROCoV-POP): a population-based study: This study presents results of a population-based household survey in Geneva, Switzerland that used weekly serosurveys to measure temporal changes in antibody presence among household members ages 5 and up. 2,766 individuals from 1,339 households participated between April 6th and May 9th. Based on survey results, the authors estimated that by late-May, only 10.8% of the Geneva population had been infected (95%CI 8.2%-13.9% — e.g. we can be 95% confident that the true estimate falls between 8.2% and 13.9%). Antibodies were less likely to be present among children ages 5–9 and adults ages 65+ compared with those 10–64 years of age. The authors conclude, “These results suggest that most of the population of Geneva remained uninfected during this wave of the pandemic, despite the high prevalence of COVID-19 in the region (5,000 reported clinical cases over <2·5 months in the population of half a million people). Assuming that the presence of IgG antibodies is associated with immunity, these results highlight that the epidemic is far from coming to an end by means of fewer susceptible people in the population.”
- Have deaths from COVID-19 in Europe plateaued due to herd immunity?: This paper explores whether declines in COVID-19 deaths in Europe can be attributed to lockdowns, social distancing, and other intervention OR to achievement of herd immunity. Using European CDC data on cumulative deaths, timing of lockdowns, and other relevant seroprevalence surveys, the authors find that herd immunity is NOT the reason for the plateau in deaths. Their reasons include:
- Different levels of mortality at time of plateau: “Under herd immunity, the cumulative mortality rate due to COVID-19 per million of the population would be expected to plateau at roughly the same level in different countries (assuming similar basic reproduction numbers). This is not what the data show. For example, in Germany, the Netherlands, and Italy, all countries with good quality health care and testing capacity, the difference in mortality is several fold, with Germany at 95 deaths per million population, the Netherlands at 332 deaths per million population, and Italy at 525 deaths per million population (as of May 17, 2020).”
- Relationship between lockdown timing and mortality: “Countries that went into lockdown earlier experienced fewer deaths in the following 6 week period. This trend is therefore inconsistent with the herd immunity explanation; however, it is exactly what one would expect under the explanation that lockdowns are curtailing transmission and deaths, making them most effective when pre-lockdown transmission is low.”
- Low levels of seroprevalence and high infection mortality rate: “A strong and consistent relationship exists between the prevalence of antibodies to SARS-CoV-2 and mortality from COVID-19 in European populations, consistent with an infection fatality rate of 0·5–1·0%…which is many times higher than seasonal influenza (<0·1%)” This matters because if herd immunity had been reached through a large proportion of the population being infected, then we would expect to see a) a much higher seroprevalence rate; b) a much lower infection fatality rate; and c) very similar seroprevalence rates across the countries.
Figure 1. Country-Level Trends in Daily New Cases
Figure 2. Trends in Daily Confirmed Deaths in European Countries
