Short time between serial COVID-19 cases may hinder containment

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 today outlines how Hong Kong protected 413 healthcare workers from infection with the novel coronavirus as they cared for patients without contracting the disease.

Serial interval estimated at 4.0 to 4.6 days

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 incubation period is the time from exposure to the virus to first symptoms.

Much spread might occur before symptoms

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.

Of the 28 infector-infectee pairs, 12 pairs were parts of family clusters.

Protecting healthcare workers against COVID-19

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.

Zero infections or deaths in hospital workers

Eleven unprotected healthcare workers out of 413 involved in treating patients with confirmed illness were quarantined for 14 days. No hospital workers were infected, and no hospital-acquired infections were identified after the first 6 weeks of the epidemic. This was despite the health system testing 1,275 patients with suspected infection and treating 42 patients with active, confirmed infection.

"Vigilance in hand hygiene practice, wearing of surgical masks in the hospital, and appropriate use of personal protective equipment in patient care, especially performing aerosol-generating procedures, are the key infection control measures to prevent nosocomial transmission of SARS-CoV-2 [the COVID-19 virus]," the authors wrote.

The investigators also collected air samples from near the mouth of a patient with a moderate viral load. The virus was not detected in any test, and tests of objects in the room detected it only on a window bench, suggesting that environmental transmission may not be as important as person-to-person transmission.

Swift public health response in Hangzhou

Also today, investigators at the First People's Hospital at Zhejiang University School of Medicine published a letter in the same journal crediting public health efforts for rapid containment of the spread of COVID-19 in Hangzhou, China.

The city's COVID-19 cases climbed from an initial 6 on Jan 19 to 169 by Feb 27. In the last week that they studied, the number of new cases decreased sharply, and only 1 case was confirmed from Feb 17 to 20, according to the authors. There were no deaths among the patients.

The researchers used a regression of log–incidence-over-time model to generate a fitted trajectory for daily incidence to prove the effects of the efforts, which began on Jan 23 with the launch of the highest level of emergency public health alert and response to limit people's movements.

On Feb 3, Hangzhou officials said only one family member was allowed to leave the family home and buy essentials outdoors every 2 days. At the same time, authorities implemented a package delivery method that involved no close contact with clients, which many express delivery companies adopted. Officials also urged employees and students to work online and arranged chartered transportation to help migrants return to their workplaces.

On Feb 11, Hangzhou implemented a system consisting of green, yellow, and red codes. People wanting to visit Hangzhou had to submit their travel history and health information online before they could do so. A green code indicated a low risk of infection, while residents with yellow or red codes had to be quarantined for 7 to 14 days and report their health condition daily before their code turned green.

The researchers said that their study shows that these efforts succeeded. "Now this health surveillance system has been applied in most cities in Zhejiang province and then would be promoted to other provinces," they wrote.

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