COVID-19 twice as contagious as previously thought – CDC study

A person with COVID-19 could infect up to 6 others – not 2 or 3 as previously thought – making measures to control the pandemic ever more critical, according to a new CDC study.

Los Alamos National Laboratory in New Mexico, US studied data from Wuhan, China and estimated the basic reproductive number (R₀) of SARS-CoV-2, the coronavirus that causes COVID-19, to be 5.7^[1]https://wwwnc.cdc.gov/eid/article/26/7/20-0282_article?deliveryName=USCDC_333-DM25287.

This is double the previously estimated R₀ of 2.2 to 2.7, meaning that the COVID-19 pandemic may be much more difficult to contain.

“Results show that quarantine and contact tracing of symptomatic persons can be effective when the fraction of unidentified persons is low,” authors Steven Sanche, Yen Ting Lin, Chonggang Xu, Ethan Romero-Severson, Nick Hengartner, and Ruian Ke write. “However, when 20% of transmission is driven by unidentified infected persons, high levels of social distancing efforts will be needed to contain the virus (Figure 6), highlighting the importance of early and effective surveillance, contact tracing, and quarantine.”

“We need to now be even more vigilant than previously thought given the higher revised R₀,” epidemiologist Eric Feigl-Ding said.

He points out that this R₀ is much higher than the recent Imperial College review of published R₀ of 3.87^[2]https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-Europe-estimates-and-NPI-impact-30-03-2020.pdf.

“Ergo, a 5.7 is on a considerably different level of infectiousness,” Dr Feigl-Ding said.

Dr Feigl-Ding explains that R₀ is the “R reproductive number at time 0 before countermeasures”.

He points out that this is not the R(effective) at current time under mitigation measures such as distancing and testing, tracing and quarantine, which are expected to slash chains of transmission.

According to the study’s authors point out that at least “82% of the population has to be immune, through either vaccination or prior infection, to achieve herd immunity to stop transmission.”

The authors of the study issue a stark warning to policy makers.

“Our results suggest that a combination of control measures, including early and active surveillance, quarantine, and especially strong social distancing efforts, are needed to slow down or stop the spread of the virus,” they write. “If these measures are not implemented early and strongly, the virus has the potential to spread rapidly and infect a large fraction of the population, overwhelming healthcare systems.”

But all hope is not lost, according to the researchers.

“Fortunately, the decline in newly confirmed cases in China and South Korea in March 2020 and the stably low incidences in Taiwan, Hong Kong, and Singapore strongly suggest that the spread of the virus can be contained with early and appropriate measures,” the Los Alamos scientists conclude.