Viral Load Drives the Transmission of COVID

Knowing what factors drive the transmission of COVID is crucial to informing schemes to contain and adapt to its spread.

There is abundant data on location and duration of exposure to COVID, but there is precious little on any other factors, the researchers noted.

The SARS-CoV-2 virus is detectable in respiratory tract samples 1-2 days before symptom onset, and can continue for several weeks afterward. But the detection of viral RNA is not necessarily related to infectiousness, and no studies had established the connection between viral load and COVID transmission, although viral load had been linked to COVID mortality.

The researchers used linked index cases of COVID to determine that viral load was the principle factor behind COVID transmission, and is the largest such study to date. This emphasises the need for initiating proper contact tracing.

The researchers conducted a post-hoc analysis of 314 cases during the initial COVID wave in Spain, of whom 90% (282) had at least one close contact, resulting in a total of 753 contacts. Viral load was the key indicator of COVID transmission.

The overall secondary attack rate (proportion of COVID-positive contacts) during the study period was 17%. However, this rate varied from 12% when the viral load of the index case was lower than 1×10⁶ copies per mL, to 24% when it was 1×10¹⁰ copies per mL or higher.

Running multivariate analyses, the researchers found no association between sex, age, diabetes, cardiovascular disease, or respiratory disease and the risk of or time to developing symptomatic COVID. There was a significant association of incubation time with initial viral load, increasing from five days among patients with a high viral load to seven days among participants with a low viral load.

The researchers wrote: “Taken together, our results indicate that the viral load, rather than symptoms, might be the predominant driver of transmission.”

They conclude that all cases should be considered potential transmitters of the virus. They also suggest viral load assessment in patients with large numbers of contacts, and risk stratification by time to symptom appearance based on viral load. 

Source: News-Medical.Net

Journal information: Marks M, et al. Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study. The Lancet Infectious Disease, 2021. DOI: https://doi.org/10.1016/S1473-3099(20)30985-3, https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30985-3/fulltext