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COVID-19 most transmissible during early stages

Tristan Manalac
08 May 2020

The coronavirus disease 2019 (COVID-19) appears to be most transmissible during the early stages of the disease, or even before symptom onset, according to a recent study.

“Our analysis of close contacts to confirmed COVID-19 cases revealed a relatively short infectious period of COVID-19 and a higher transmission risk around the time of symptom onset of the index case, followed by a lower transmission risk at the later stage of disease,” researchers said.

Researchers enrolled 100 confirmed COVID-19 patients (median age, 44 years; 56 percent male) with 2,761 close contacts. From here, 22 secondary cases of COVID-19 were identified through contact tracing, with an infection risk of 0.8 percent. The secondary clinical attack rate was calculated to be 0.7 percent. There were nine asymptomatic cases, none of whom transmitted the disease to a secondary case. [JAMA Intern Med 2020;doi:10.1001/jamainternmed.2020.2020]

More than a quarter of close contacts were encountered through the healthcare setting. In comparison, household and nonhousehold family contacts comprised 5.5 percent and 2.8 percent of the close contact network, respectively. From the 22 secondary cases, the researchers obtained a median incubation estimate of 4.1 days.

All secondary cases were exposed to the index cases before the sixth day of symptom onset. Notably, the secondary clinical attack rate was much higher among patients who were exposed to the index case within 5 days of symptom onset. The same was true for those who had been exposed before index symptom onset. The secondary clinical attack rate in this group was 1.0 percent.

There was zero transmission of the virus among those who had been exposed beyond 6 days after the initial onset of symptoms.

“The observed decreasing transmission risk over time for COVID-19 was in striking contrast to the transmission pattern of severe acute respiratory syndrome (SARS), in which the transmission risk remained low until after day 5 of symptom onset in the index cases,” the researchers said. [Science 2003;300:1966-1970]

The researchers also found high secondary clinical attack rates in household (4.6 percent) and nonhousehold family (5.3 percent) contacts. The high transmissibility during the early stages of the disease remained true in both subgroups.

Attack rates were also higher for older participants and for those who had contact with a patient with severe COVID-19.

“The observed short duration of infectiousness with lower risk of transmission 1 week after symptom onset has important implications for redirecting the efforts to control COVID-19,” the researchers said, pointing out that because early symptoms are mild and nonspecific, the disease is identified too late, when the patient starts developing more serious symptoms and presents to the hospital.

“In this case, hospitalization would not be helpful for isolation and reducing transmission and should be only for patients whose clinical course is sufficiently severe,” they added.

The present study thus shows that a better understanding of the disease’s transmission dynamics and duration could help streamline containment efforts.

“The pattern of high transmissibility near and before symptom onset and the likely short infectious period of the virus could inform control strategies for COVID-19, as well as additional studies to fully elucidate the transmission dynamics of the virus,” the researchers said.

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