There is no separate R0 for young adults. The general population average applies to them too. However, the number of people a given person infects can be above or below the average, depending on how many contacts they have and how mobile they are.
The averages calculated for each variant are as follows:
- Early phase (Wuhan strain): A review of 12 studies found a mean of 3.28 and a median of 2.79. Estimates for Italy vary by study, roughly between 2.4 and 3.3. For comparison, the median for seasonal influenza is about 1.3. So in the early phase, a person with Covid infected more than twice as many people as a person with the flu.
- Delta: Mean R0 of 5.08 (range 3.2–8).
- Omicron: Mean R0 of 9.5. Estimates span a very wide range (5.5–24).
What would have happened without measures?
Assuming no immunity and no measures, one infected person would have infected on average 2 to 3 people in the early phase, about 5 with Delta, and about 8 to 10 with Omicron.
What actually happened?
In real life, what matters is the effective reproduction number (Rt or Re), measured at a given point in time. Masks, vaccines, immunity from previous infections and reduced contact lower this number. When Rt falls below 1, the epidemic starts to shrink. For example, while the mean R0 of Omicron was 9.5, its mean effective reproduction number was calculated at about 3.4.
The average was not the same for everyone
Transmission was not evenly distributed. Most infected people infected no one or only one other person. By contrast, about 10–20 percent of cases caused about 80 percent of transmissions. These superspreaders sometimes infected 10 or more people on their own. This is why the average could differ greatly from the number of people any one individual actually infected.
Sources:
- Liu et al. 2020, Journal of Travel Medicine: https://academic.oup.com/jtm/article/27/2/taaa021/5735319
- D’Arienzo and Coniglio 2020 (Italy): https://pmc.ncbi.nlm.nih.gov/articles/PMC7148916/
- Lombardy early-phase study: https://pmc.ncbi.nlm.nih.gov/articles/PMC8605863/
- Liu and Rocklöv 2021 (Delta): https://pmc.ncbi.nlm.nih.gov/articles/PMC8436367/
- Liu and Rocklöv 2022 (Omicron): https://academic.oup.com/jtm/article/29/3/taac037/6545354
- Biggerstaff et al. 2014 (influenza): https://stacks.cdc.gov/view/cdc/25291/cdc_25291_DS1.pdf
- Superspreading, PLOS Biology: https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3000897
- Superspreading, PNAS: https://www.pnas.org/doi/10.1073/pnas.2016623118