A mass screening program of more than 10 million Wuhan residents identified 300 asymptomatic cases, but none of them were infectious – according to a study involving the University of East Anglia.
The mass-testing project took place over two weeks in late May – after the city’s strict lock-up was lifted in April.
The study did not find any ‘viable’ virus in the asymptomatic cases and the close contacts of these positive asymptomatic cases did not test positive.
But the research team warns that their findings do not show that asymptomatic carriers cannot transmit the virus.
Instead, harsh non-pharmaceutical interventions such as wearing masks, hand washing, social isolation and locking have helped to reduce the virulence of Covid-19.
The study, published in Nature Communications, was led by researchers at Huazhong University of Science and Technology (HUST) in Wuhan, China – in collaboration with researchers at Norwich UEA Medical School.
Wuhan was the city most affected in China by the Covid-19 pandemic. After stringent non-pharmaceutical interventions and lock-down measures from 23 January 2020, the city lockdown was lifted on 8 April 2020. To assess the post-lockout risk of Covid-19 in the city, a mass screening project was nearing completion to 10 million Wuhan residents conducted between May 14 and June 1. This screening program identified 300 asymptomatic cases. But the virus cultures did not identify any viable virus in the asymptomatic cases identified. This means that these people were not likely to infect anyone else. “
Fujian Song, Professor, Norwich Medical School, University of East Anglia
The asymptomatic incidence was found among 10 to 89 year olds, however, the asymptomatic positive rate was lowest among children and adolescents under 17 years of age and highest among over 60s.
Further swab tests of 1174 close contacts of these 300 asymptomatic positive cases were all negative.
Professor Song said: “What this tells us is that the prevalence of Covid-19 infections is very low five to eight weeks after the conclusion of the lock-up process in Wuhan.
“This work confirms that Covid-19 transmission can be successfully managed through well-implemented pharmaceutical interventions, including face covering, hand hygiene, safe social isolation, contract tracking, and locking restrictions. , wearing a mask remains common in public places now in Wuhan. “
However, the research team states that it is important to stress that the results of this study should be interpreted correctly.
Professor Song said: “The asymptomatic cases identified in the screening program in Wuhan were truly asymmetric, as none showed clinical symptoms before or during their subsequent isolation.
“But there is plenty of evidence elsewhere that shows that people infected with Covid-19 may be transient and infectious before going on to develop symptoms.
“It is also very important to say that these asymptomatic outbreaks were identified shortly after a very tight lockdown in Wuhan that lasted for more than 70 days. By that time, the Wuhan epidemic was effectively brought to bear .
“The virulence of Covid-19 may be weakening over time. And it is likely that the viral load of Wuhan asymptomatic cases may be low, compared with cases in locations with a high level of virus transmission.
“Antibody testing showed that almost two-thirds of asymptomatic cases had previously received Covid-19.
“Because the risk of residents becoming infected in the community has been greatly reduced, when susceptible residents are exposed to low doses of virus, they can tend to be disproportionate as a result of their own immunity.
“Therefore, it would be problematic to apply the results of our research to countries where Covid-19 outbreaks have not been successfully controlled.
“In fact, the existence of asymptomatic cases remains a concern even in Wuhan. It is too early to be complacent, due to the existence of asymptomatic positive cases and a high degree of tendency among residents in Wuhan.
“Public health measures for the prevention and control of the Covid-19 epidemic should be maintained, including wearing masks, maintaining a safe social distance in Wuhan. And vulnerable populations with weak immunity or co-morbidity, or both, should continue to be properly shaded. “
‘Post-lock SARS-CoV-2 nucleic acid screening in nearly ten million Wuhan, China residents’ is published in the journal Nature Communications.
University of East Anglia
High, S., et al. (2020) Post-lock SARS-CoV-2 nucleic acid screening in nearly ten million Wuhan, China residents. Communicating Nature. doi.org/10.1038/s41467-020-19802-w.