Who is the most contagious?

At the level of the overall population, the R₀ reproduction rate tracks the transmission dynamics of COVID-19. At an individual level, however, quantifying and predicting transmission risks is more difficult, especially when symptoms are not yet visible, or absent, or moderate (we use the acronym PAMS: pre-symptomatic, asymptomatic, and mildly-symptomatic). In addition, contagiousness varies, depending on the person, the length of the illness or the presence of different variants. The most reliable parameters are the measurement of viral load (viral RNA levels) and the in vitro evaluation of infectious potential (infectivity) of samples. These parameters allow the risk of transmission in certain population groups or within different variants to be evaluated, so that appropriate health measures can be implemented. In PAMS patients, this data is rarely collected and evaluation may be difficult when symptoms do finally emerge.

To better understand the contagiousness of SARS-CoV-2, German researchers (University of Berlin-Charité) evaluated viral load, infectivity and viral sequences from 25,381 COVID-19 patients (February 2020 – April 2021). The correlations between these parameters were established by mathematical models (Bayesian linear regressions) and then adjusted according to other parameters such as age, gender or clinical status, or the logistics and protocols used for the analysis.

Amongst these patients, 37.5% had been hospitalised, with an average age of 63,2; 24,1% were PAMS with an average age of 38; and 38,4%, with an average age of 49,1 were classified as “other”. Around 6% of them were infected with the UK variant. The average level of viral load (log₁₀ RNA copies/swab) was 6 for the hospitalised patients and 6,9 for the PAMS, and 7,38 for those carrying the UK variant. The viral load was reduced (up to -0,96) in patients younger than 20. The probability of isolating and amplifying the virus in vitro was higher in PAMS than in the hospitalised patients, and 2,6 times higher for those infected with the UK variant. The correlation between levels of viral load and the fact that this variant is more transmissible was thus proven.

On average, it takes between 1 and 3 days to reach maximum contagiousness after symptoms appear. A minority (8,78%) of patients were classed as “very infectious”; 36,09% of them were PAMS. Amongst the 4 344 patients from whom several samples had been taken over time (80% hospitalised), it was estimated that from the moment that the samples showed them to be infectious, it took only 4,31 days for PAMS patients to reach peak viral charge, while it took 7,4 days for other patients. Amongst these subjects, 22,14% had had their 1^st PCR test 1,4 days on average before the viral load peak was attained, while the others had had their 1^st test 9,8 days after. It takes 1,8 days for samples to move from a non-infectious state to the peak level of infection.

This study is limited in many ways, principally due to the lack of regularity of sampling, and because asymptomatic cases were often detected too late, which is the heart of the problem discussed here. For several viruses, the relation between viral load and transmission is difficult to establish. Compared to influenza, COVID-19’s contagiousness appears earlier and ends later, viral load is more substantial, and rates of transmission by PAMS subjects is higher. This study indicates that PAMS are in general just as contagious as hospitalised patients and that they can cause just as many emergency situations while they mix freely with others. COVID-19’s period of high contagiousness appears several days after the appearance of symptoms (when they are present). This data also corroborates other studies suggesting that 10-15% of COVID-19 cases are responsible for 80% of transmissions. The difficulty remains to link contagiousness to transmissions in vivo, which are very dependent on life style. The authors of this study concluded that measures of social distancing and mask wearing have significantly limited the appearance of new outbreaks.