Transmission of SARS-CoV-2 between different the ages of groups renders age-based interventions ineffective
Many countries have implemented non-pharmaceutical interventions (NPIs) for example, social distancing requirements or lockdowns in response to the coronavirus pandemic 19 (COVID-19). Although these measures reduced the strain on the healthcare system in the absence of vaccines, they were associated with significant socioeconomic costs. This led to a resurgence of the debate on whether COVID-19 control strategies should target at-risk populations or disease transmitters.
The authors of a recent Nature Communications review critically evaluated the dynamics of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) across different age groups in metropolitan France between June 15, 2020 and September 28 in 2020. In the end, they recommended that it is not appropriate to replace or reduce the severity of the lockdown because there is a significant porosity in SARS-CoV-2 transmission across age groups.
Study: Transmission of SARS-CoV-2 in France across different age groups and implications for reducing the risk. Image credit Ocskay Mark/Shutterstock.com
Information about the study
The research team developed an analytical framework to represent the intricate patterns of SARS-CoV-2 infections across different age groups. In addition, they investigated the dynamics of the infection and hospitalizations. They analyzed age-stratified cases and hospitalization data from Auvergne-Rhone-Alpes, one of the first regions in France to experience the second wave of COVID-19 cases.
Researchers also replicated the same analysis for 13 other regions of metropolitan France. Based on these analyses, they quantified the contribution of each age group to the COVID-19 transmission, which further enabled the evaluation of various age-specific intervention strategies that were implemented in absence of vaccines.
Comparing transmission patterns across different age groups
Among the symptomatic individuals between the ages of 20-29 years, the proportion of positive reverse transcription-polymerase chain reaction (RT-PCR) tests increased from 3.2% to 12.9% between July 27, 2020, and August 17, 2020, followed by a rise in positivity rates and hospital admissions in those older than 80 years.
The pattern was similar in metropolitan France with a mean delay between the rise in positive tests among symptomatic people and COVID-related hospitalizations for the 20-29-year-old age group and those who are over 80 years of age.
Estimating the contributions of different age groups to transmission
The concept of effective reproduction numbers (R ef) was used to quantify the impact of interventions. R eff refers to the average number of individuals infected by an Index case that account for the buildup of immunity.
If the rate of transmission remains constant, R eff decreases as the percentage of individuals susceptible to SARS-CoV-2 infection decline. If the entire population is susceptible to SARS-CoV-2, the reproduction number (R i) indicates the average number of infections that can be resulted from a single instance of intervention.
During the lockdown period starting May 11, 2020 and lasting until July 8, 2020, the R i increased in Auvergne-Rhone Alpes from 0.71 to 0.90 and then to 1.46 during the second wave of the pandemic that ran from July 9, 2020, to September 28, 2020. During this time, the number of effective contacts was highest for those aged 20-29. The proportion of all age groups to transmission is between 17% to 37% less than that of those aged 20-29.
Strategies to protect the elderly population
The researchers looked into whether the protection of individuals aged 70 years or above would be adequate to keep R efficacy greater than or equal to 1.3-1.5 while allowing for more relaxed control measures. They decreased the effective contacts for the target age group by half.
The results for R eff values 1.3-1.5 revealed 53-116 admissions to hospitals at peak and 664-1074 deaths per million. Reducing the control measures and bringing R eff value to 1.8 increased the number of hospitalized patients to 233 per million and the total number of deaths to 1,646 per million at the peak.
The study showed that COVID-19 sufferers older than 70 were still being admitted to hospitals despite reductions in their numbers.
Evaluation of transmission dynamics across all 13 regions of metropolitan France
The R eff value must be kept at 1 for the pandemic to remain manageable, which requires efforts from all age groups. In 10 out of 12 regions of Metropolitan France, to keep R eff close to 1, decreasing the number of contacts that are effective for those who contribute most to transmission was beneficial. The most at-risk groups were those with higher R eff values, which could lead to an COVID-19-related crisis in hospitals.
The highest proportions of people who are over 80 in metropolitan France are located in Nouvelle-Aquitaine as well as Provence-Alpes-Cote d’Azur and Provence-Alpes-Cote-d’Azur. In these regions, considering that the R eff was low, it was beneficial to target older individuals to maximize the reduction in deaths, as indicated by the lower numbers of the number of contacts that are effective for those who are 80 years old or older at 1.55 for Nouvelle-Aquitaine and 2.38 for Provence-Alpes-Cote-d’Azur.
The study’s findings show that COVID-19 is transmissible across all age groups and that preventive measures to fight the COVID-19 pandemic should , therefore, equally target all age groups.
This will help address social and ethical concerns that arise from employing different strategies for the young and elderly. Furthermore, the increased compliance of people from all age groups to these strategies will increase their overall impact.
- Tran Kiem, C., Bosetti, P., Paireau, J., et al. (2021). SARS-CoV-2 transmission across different age groups in France and implications for control. Nature Communications. doi:10.1038/s41467-021-27163-1
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