SARS-CoV-2 transmission across age groups makes age-based interventions ineffective

Many countries have implemented various nonpharmaceutical interventions (NPIs) such as social distancing mandates and lockdowns in response to the coronavirus disease 19 (COVID-19) pandemic. Although these measures reduced stress on the healthcare system in the absence of vaccines, they were associated with major socioeconomic costs. This led to a revival of the debate on whether COVID-19 control strategies should be targeting at-risk individuals or disease transmitters.

The authors of a recent Nature Communications review critically evaluated the transmission dynamics of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) across various ages in metropolitan France between June 15, 2020, and September 28, 2020. Ultimately, these researchers recommended that it would be inadequate to substitute or ease down a lockdown, as there is substantial porosity of SARS-CoV-2 transmission across age groups.

Study: SARS-CoV-2 transmission across age groups in France and implications for control. Image Credit: Ocskay Mark /

About the study

The research team developed a modeling framework representing the complex patterns of SARS-CoV-2 infection across age groups, wherein they studied the infection dynamics and hospitalizations. They analyzed age-stratified cases and hospitalization data from Auvergne-Rhône-Alpes, one of the first regions in France to experience the second wave of COVID-19 cases.

The researchers also reproduced the same analysis for 13 other regions in 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 different age-targeted intervention measures implemented in the absence of vaccines.

Comparing transmission dynamics across 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 across metropolitan France, with a mean lag of four weeks between the increase in the proportion of positive tests among symptomatic individuals and COVID-related hospital admissions in the 20–29 year age group and those older than 80 years.

Estimating the contribution of different age groups to transmission

The impact of interventions was quantified using the concept of effective reproduction numbers (Reff). Reff is the average number of individuals infected by an index case accounting for the build-up of immunity.

If the transmission rate is unchanged, Reff declines as the proportion of individuals susceptible to SARS-CoV-2 infection decline. If the whole population is prone to SARS-CoV-2 infection, the intervention reproduction number (Ri) is the average number of infections occurring from a single case under a set of interventions.

During the lockdown period starting May 11, 2020, and ending July 8, 2020, Ri increased in Auvergne-Rhône-Alpes from 0.71 to 0.90 and further to 1.46 during the second wave of the pandemic from July 9, 2020, to September 28, 2020. During this time, the number of effective contacts was highest in individuals aged 20–29. The contribution of all other age groups to transmission compared to those aged 20–29 is between 17% and 37% lower.

Effect of strategies shielding the elderly population

The researchers examined whether protecting individuals aged 70 years or above would be adequate to maintain Reff greater than or equal to 1.3–1.5 while relaxing control measures. They reduced the number of effective contacts of the target age group by 50%.

For Reff values between 1.3–1.5, the results showed 53–116 per million daily hospital admissions at the peak and 664–1074 deaths per million. Relaxing the control measures and bringing Reff value to 1.8 increased the daily number of hospitalized patients to 233 per million and the overall number of deaths to 1,646 per million at peak.

This indicated that the reductions did not stop the surge of COVID-19 patients in hospitals in individuals over the age of 70 when control measures were relaxed.

Evaluating the transmission dynamics across all the 13 regions of metropolitan France

The Reff value needs to be maintained 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 Reff close to 1, reducing the effective contacts of those that contribute the most to the transmission was beneficial. For larger values of Reff leading to a COVID-19-induced crisis in hospitals, those with the highest risk of the severe outcome were targeted.

In metropolitan France, Nouvelle-Aquitaine and Provence-Alpes-Côte d’Azur have the highest proportion of individuals over the age of 80. In these regions, given that the Reff was low, it was beneficial to target older individuals to maximize the reduction in deaths, as indicated by the lower values of the number of effective contacts in those 80 years or older at 1.55 for Nouvelle-Aquitaine and 2.38 for Provence-Alpes-Côte d’Azur.


The study results establish that COVID-19 is transmissible across all age groups and that preventive measures to control the COVID-19 pandemic should therefore equally target all age groups.

This will help address the ethical and social concerns about using different strategies for the young and elderly. Furthermore, the increased compliance of individuals of all age groups to these measures would enhance their overall impact.

Journal reference:
  • Tran Kiem, C., Bosetti, P., Paireau, J., et al. (2021). SARS-CoV-2 transmission across age groups in France and implications for control. Nature Communications. doi:10.1038/s41467-021-27163-1
Neha Mathur

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.


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