Contact tracing different age groups during the first five waves of the COVID-19 pandemic: a retrospective study from south-west Germany (Preprint)

Abstract

Contact tracing was used in many countries during the initial waves of the COVID-19 pandemic to prevent disease spread, reduce mortality and avoid the overburdening of health care systems. In many countries, including Germany, entirely new systems were employed to trace and quarantine potentially infected persons.

Using data collected in the Rhine-Neckar district and Heidelberg city (RNK/HD) in south-west Germany (population 706 974), this study examines differences and the overall effectiveness and efficiency of contact tracing within different age groups and waves of the pandemic.

From 27 January 2020 until 30 April 2022, the Heidelberg and Rhein-Neckar Public Health Authority routinely collected data on COVID-19 infection, quarantine and death. To assess variations in the proportion, risk and relative risk of infection, quarantine and deaths, data was grouped into age groups (young: 0-19; adult: 20-65; and elderly: >65 years) and by date of pandemic waves. The overall effectiveness and efficiency of contact tracing was determined for different age groups and timepoints by calculating the quarantine sensitivity (proportion of infected population captured in quarantine) and positive predictive value (PPV; proportion of quarantined population that were infected).

Within the study period and location, 28.0% of the population tested SARS-CoV-2 positive, 11.1% were quarantined, and 0.123% died following an infection. Compared to adults, the relative risk of infection was lowest for the elderly (RR: 0.400; 95%CI: 0.394-0.406), and initially lower for young people before increasing (first wave RR: 0.523; 95%CI: 0.458-0.597; all waves RR: 1.35; 95%CI 1.34-1.36). Over 90% of COVID-19 associated deaths occurred amongst elderly people, with no associated deaths amongst young people. Elderly people had the lowest risk of quarantine compared to adults (RR: 0.474; 95%CI: 0.462-0.487), while young people had the highest risk (RR to adults: 2.69; 95%CI: 2.65-2.72). During the first three waves of the pandemic, contact tracing captured from between 28.0% to 46.4% of all infections in quarantine (sensitivity), with 5.91% to 23.4% of the quarantined population testing positive for COVID-19 (PPV).

Data from RNK/HD supports national and international research showing that quarantine was an important and effective infection control measure prior to the introduction of the vaccine. Increased use of quarantine in the first three waves of the pandemic was consistent with the established risks of COVID-19 infection and death. However, the disproportional use of quarantine was inconsistent with the risks of infection or death in specific age groups, and unsupported by emerging knowledge of transmission pathways, incidence of long-COVID or other risks. More generally, however, quarantine sensitivity and PPV measurements demonstrate how contact tracing systems were increasingly effective in capturing more of the infected population in quarantine, and increasingly efficient in raising the proportion of infections within the quarantine population.