The reason why China’s Covid-19 cases have no symptoms

Epidemiologists are trying to explain, during the outbreak Covid-19 the largest in China in the past two years, the rate of asymptomatic cases is so high. They also want to assess what that means for China’s future Covid-19 containment strategy.

The number of community Covid-19 cases in Shanghai city reached a record high, more than 4,000 people on March 29, but only 2.1% had symptoms. The average 7-day rate of symptomatic cases is about 1.6%.

People receive food delivered by a delivery person during the outbreak in Shanghai.

Outbreaks in many countries have proven Omicron to be less deadly than previous variants, but the rate of symptomatic infection remains relatively high compared with China’s numbers. In the UK, prevalence of symptomatic Omicron infection ranges from 50% to 75%.

Here are some explanations for why the number of asymptomatic cases in Shanghai is so high.

Screening test

China is the rare major country to carry out mass screening tests, aiming to detect more asymptomatic cases, although more symptomatic cases are likely to be detected.

Adrian Esterman, an expert in biostatistics at the University of South Australia, said: “Certainly, high levels of testing will find more asymptomatic cases.”

In other countries, many people who test positive at home do not report it to the authorities. Official data also shows that the number of Covid-19 infections decreased as the number of tests decreased.

On March 28 alone, Shanghai conducted more than 8 million tests at more than 60,000 locations.

Viruses are less dangerous, vaccination rates are higher

China’s uncompromising response to the new variant is partly due to uncertainty about the level of immunity and resistance of the population after nearly 2 years of the pandemic.

Expert Zhang Wenhong assessed that, although the Omicron variant is more difficult to eliminate, it is clearly less scary than previous SARS-CoV-2 virus strains.

Chinese experts, including Zhang Boli, who advises the government on Covid-19 treatment, say Omicron’s lower pathogenicity, combined with the country’s relatively high vaccination rate, has can reduce the number of symptomatic infections.

However, vaccination levels in South Korea and Singapore are higher than in China, but the two countries still have more symptomatic patients.

Early detection

Zhang also said that the large proportion of Covid-19 cases without symptoms is not necessarily due to the impact of the virus.

The high number may be due to the early detection of the patient in China. Therefore, the authorities will quickly isolate the infected cases before they show symptoms.

Wu Zunyou, head of the epidemiology team of the Chinese Center for Disease Control, said, “no symptoms” is not a fixed state. People can start showing signs within a few days and still need to pay attention to the rate of infection.

An Yen (According to Reuters)