Transmission of SARS-CoV-2 across age groups renders age-based interventions ineffective


Many countries have implemented various non-pharmaceutical interventions (NPI) such as social distancing warrants and lockdowns in response to the coronavirus disease 19 (COVID-19) pandemic. Although these measures reduced the stress on the health system in the absence of vaccines, they were associated with significant socio-economic costs. This has led to a resumption of debate on whether COVID-19 control strategies should target people at risk or disease transmitters.

The authors of a recent Nature Communication review critically assessed the transmission dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at different ages in mainland France between June 15, 2020 and September 28, 2020. Ultimately, these researchers recommended that ‘it would be insufficient to replace or ease a lockdown, as there is significant porosity in the transmission of SARS-CoV-2 in all age groups.

To study: Transmission of SARS-CoV-2 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 in all age groups, in which they studied the dynamics of infection 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 mainland France. On the basis of these analyzes, they quantified the contribution of each age group to the transmission of COVID-19, which also made it possible to assess the different intervention measures targeted at age implemented in the absence of vaccines.

Comparison of transmission dynamics between age groups

Among symptomatic individuals aged 20 to 29, the proportion of positive reverse transcription-polymerase chain reaction (RT-PCR) tests increased from 3.2% to 12.9% between July 27, 2020 and July 17, 2020. August 2020, followed by an increase in positivity rates and hospital admissions among those over 80.

The pattern was similar across metropolitan France, with an average lag of four weeks between the increase in the proportion of positive tests in symptomatic individuals and COVID-related hospitalizations in the 20-29 age group and those of over 80 years old.

Estimation of the contribution of different age groups to transmission

The impact of the interventions was quantified using the concept of effective reproduction number (Reff). Reff is the average number of individuals infected with an index case, which explains the accumulation of immunity.

If the transmission rate is unchanged, Reff decreases as the proportion of individuals susceptible to infection with SARS-CoV-2 decreases. If the entire population is prone to SARS-CoV-2 infection, the intervention’s reproduction number (RI) is the average number of infections occurring from a single case across a set of interventions.

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

Effect of protection strategies for the elderly population

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

For Reff values ​​between 1.3 and 1.5, the results showed 53 to 116 per million maximum daily hospitalizations and 664 to 1074 deaths per million. Relaxation of control measures and contribution of Reff The value at 1.8 increased the daily number of inpatients to 233 per million and the total number of deaths to 1,646 per million at the peak.

This indicated that the cuts did not stop the surge of COVID-19 patients in hospitals among people over 70 when the controls were relaxed.

Evaluate transmission dynamics across all 13 regions of mainland France

The Reff the value needs to be kept at ~ 1 to keep the pandemic manageable, requiring efforts from all age groups. In 10 out of 12 regions of mainland France, to keep Reff close to 1, reducing effective contact with those who contribute the most to transmission has been beneficial. For higher values ​​of Reff Leading to a COVID-19-induced crisis in hospitals, those at the highest risk of becoming severe have been targeted.

In mainland France, Nouvelle-Aquitaine and Provence-Alpes-Côte d’Azur have the highest proportion of individuals over 80 years of age. In these regions, since the Reff was low, it was beneficial to target the elderly to maximize the reduction in deaths, as indicated by the lower values ​​of the number of effective contacts in people 80 years or older than 1.55 for Nouvelle-Aquitaine and 2, 38 for Provence-Alpes-Côte d ‘Azur.


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

This will help address the ethical and social concerns associated with using different strategies for young and old. In addition, increased compliance of individuals of all age groups with these measures would enhance their overall impact.

Journal reference:

  • Tran Kiem, C., Bosetti, P., Paireau, J., et al. (2021). Transmission of SARS-CoV-2 across age groups in France and implications for control. Natural communications. doi: 10.1038 / s41467-021-27163-1