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Explainer | Why is Shanghai seeing so many asymptomatic Covid-19 infections?
- China is in the grip of an Omicron wave, but about 70 per cent of cases reported in March have not had any symptoms
- Virus traits, immunity, vaccination rate and early detection through mass testing are all factors, according to health official
Reading Time:4 minutes
Why you can trust SCMP
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Since the early stage of the pandemic, it has been clear that some people infected with Sars-CoV-2 – the virus that causes Covid-19 – can have no symptoms at all.
It is not clear what proportion of cases they are because study findings vary, but China has seen a large number of asymptomatic infections during the current Omicron wave.
Of the 103,965 locally acquired cases reported in March, only 3,046 had symptoms, according to National Health Commission data. And most of the asymptomatic infections were reported in Shanghai.
Unlike other countries, China does not count asymptomatic infections in its official Covid-19 tally – it only includes people who have symptoms. But asymptomatic case numbers are still reported, and those who test positive without symptoms are still sent into isolation.
Here is what we know about asymptomatic infections, why they are surging in China, and how it has responded.
What does it mean to be asymptomatic?
China’s definition of an asymptomatic Covid-19 case is the same as the World Health Organization’s and other countries’ – a positive result (from a PCR test in China) but without any of the symptoms listed in its diagnosis and treatment guidelines.
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