Short time between serial COVID-19 cases may hinder containment

March 9, 2020

A study involving 28 COVID-19 patients in Japan has shown that the virus's serial interval—the time between successive cases—is close to or shorter than its median incubation period, suggesting pre-symptomatic transmission may play a key role in the outbreak and case isolation alone might not be as effective as hoped.

Also, a separate study outlines how Hong Kong protected 413 healthcare workers from infection with the novel coronavirus as they cared for patients without contracting the disease.

In the Japanese study, published yesterday in the International Journal of Infectious Diseases, investigators calculated that the time from symptom onset in a primary COVID-19 patient to symptom onset in secondary patients, or the serial interval, was 4.0 to 4.6 days.

Led by researchers at Hokkaido University in Sapporo, the study, though small, is important because serial interval enables identification of epidemiologic links between cases and is an important parameter in epidemic transmission models to inform infection control methods.

"When the serial interval is shorter than the incubation period, pre-symptomatic transmission is likely to have taken place and may even occur more frequently than symptomatic transmission," the authors wrote.

The researchers gathered dates of illness onset of primary patients (infectors) and secondary patients (infectees) from published research articles and case reports. They subjectively ranked the legitimacy of the data, analyzed both the full dataset (28 patients) and a subset of pairs that had highest certainty in reporting (18 patients) and then adjusted for right truncation of the data because the epidemic is still growing.

They estimated the median serial interval at 4.0 days (95% credible interval [CrI], 3.1 to 4.9). When the data was limited to only the most certain pairs, the median serial interval was estimated at 4.6 days (95% CrI, 3.5 to 5.9). "This suggests that a substantial proportion of secondary transmission may occur prior to illness onset," the authors wrote.

They add, "The COVID-19 serial interval is also shorter than the serial interval of severe acute respiratory syndrome (SARS), indicating that calculations made using the SARS serial interval may introduce bias."

Because of the shorter serial interval, "contact tracing methods must compete against the rapid replacement of case generations, and the number of contacts may soon exceed what available healthcare and public health workers are able to handle," they wrote.

A robust, multifaceted response to the COVID-19 outbreak protected healthcare workers against the virus in a Hong Kong hospital, according to the second study, published today in Infection Control & Hospital Epidemiology.

Led by researchers at Queen Mary Hospital in Hong Kong, the study describes the hospital's infection control response in the first 42 days after a cluster of pneumonia in Wuhan, China, was reported on Dec 31.

The hospital, like other public hospitals in Hong Kong, immediately stepped up its infection control procedures using enhanced laboratory surveillance, early airborne infection isolation, rapid molecular diagnostic testing, and contact tracing for healthcare workers who had been unprotected against exposure.

Workers were educated about personal protective equipment, infection control, and hand hygiene in staff forums and in-person sessions. When screening identified a patient infected with the coronavirus, he or she was immediately isolated in an airborne isolation room or a ward with at least one meter of space between patients.

CIDRAP has the story.

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