The transmission rate of coronavirus disease 2019 (COVID-19) was 1%-5% among 38,000 Chinese people in close contact with infected patients, according to the chief epidemiologist of the Chinese Centers for Disease Control and Prevention, Beijing, Zunyou Wu, MD, PhD, who gave an update on the epidemic at the Conference on Retroviruses & Opportunistic Infections.

The rate of spread to family members – the driver of the infection in China – was 10% early in the outbreak, but fell to 3% with quicker recognition and isolation. The overall numbers are lower than might have been expected, and an important insight for clinicians trying to contain the outbreak in the United States.

Patients were most infectious at the onset of symptoms, when they spiked a fever and started coughing, but their ability to spread the infection dropped after that, Dr. Wu and others said at a special COVID-19 session at the meeting, which was scheduled to be in Boston, but was held online instead because of concerns about spreading the virus. The session has been posted.

Transmission from presymptomatic people is rare. Shedding persists to some degree for 7-12 days in mild/moderate cases, but 2 weeks or more in severe cases.

Dr. Wu said the numbers in China are moving in the right direction, which means that containment efforts there have worked.

The virus emerged in Wuhan, the capital of Hubei province in central China, in connection with a wildlife food market in December 2019. Bats are thought to be the reservoir, with perhaps an intermediate step between civet cats and raccoon dogs. Officials shut down the market.

Essentially, the entire population of China, more than a billion people, was told to stay home for 10 days to interrupt the transmission cycle after the virus spread throughout the country in a few weeks, and almost 60 million people in Hubei were put behind a cordon sanitaire, where they have been for 50 days and will remain “for a while,” Dr. Wu said.

It’s led to a steep drop in new cases and deaths in China since mid-February; both are now more common outside China than inside, and international numbers are lower than they were at the peak in China.

