New clues have emerged about how and when the novel SARS-CoV2 coronavirus transmits.

Not only was viral load similar in an asymptomatic patient versus those with symptoms, but it was higher in the nose than in the throat, reported Jie Wu, PhD, of Guangdong Provincial Center for Disease Control and Prevention in Guangzhou, China, and colleagues.

This suggests SARS-CoV2 has a shedding pattern closer to influenza, and one different than SARS, or SARS-CoV, they wrote in a Correspondence in the New England Journal of Medicine.

They noted the differences to SARS, which was "associated with modest viral loads in the respiratory tract early in the illness, with viral loads peaking approximately 10 days after symptom onset."

Wu and colleagues examined SARS-CoV2 viral load data from nine men and nine women (median age 59) in Zhuhai (adjacent to Macau in China's south) in two family clusters. Four patients had secondary infections, including one who never had symptoms but was a close contact of an existing case.

During Jan. 7-26, 14 of these individuals had recently returned from Wuhan with fever and were diagnosed with laboratory confirmed COVID-19, the disease caused by SARS-CoV2. Thirteen had evidence of pneumonia on CT scans, and three were admitted to ICUs, while the others had "mild-to-moderate illness."

Researchers examined 72 nasal swabs and 72 throat swabs from the 17 symptomatic patients, and found viral loads were highest soon after symptom onset, and more in the nose than the throat. Swabs were also collected from the asymptomatic patient, with a viral load in the same range as in those showing symptoms.

The latter finding suggests "transmission may occur early in the course of infection," Wu and colleagues concluded.

"Identification of patients with few or no symptoms and with modest levels of detectable viral RNA in the oropharynx for at least 5 days suggests that we need better data to determine transmission dynamics and screening practices," they wrote.

A Call for More Testing, Surveillance

Marc Lipsitch, DPhil, of Harvard T.H. Chan School of Public Health in Boston, and two colleagues -- one from Pfizer's vaccine division and another from Merck -- argued in an accompanying commentary that viral testing needs to be expanded from clinical into broader public health settings.

"Although this approach may result in many negative test results and therefore appear 'wasteful,' such set-aside capacity will permit a far greater understanding of the spread of the epidemic and wiser use of resources to combat it," they wrote.

They discussed how increased testing in "unexplained clusters or severe cases of acute respiratory infection," regardless of travel history, could detect transmission chains that have been missed. Lipsitch and colleagues pointed to the case of Singapore, which has been unable to link a number of its cases to either travel to China or known cases, despite having "one of the world's best public health systems."

On a broader note, the commentary authors called for more studies, in particular to discover the full spectrum of disease severity; SARS-CoV2's transmissibility; patient characteristics that affect risk for transmission; and the risk factors for severe illness and death. Tools used to gather this data include household studies, community studies, and case-control studies.

Lipsitch and colleagues also advocated putting infrastructure into place for better surveillance, similar to that used in the 2009 influenza pandemic. This could involve "using existing surveillance systems or designing surveys to ascertain each week the number of persons with a highly sensitive but nonspecific syndrome ... and testing a subset of these persons for the novel coronavirus," they said.

"Electronic laboratory reporting will dramatically improve the efficiency of this and other public health studies involving viral testing," they wrote.