The Chinese city of Wuhan will go down in history as the first place in the world to report patients with Covid-19. In addition, between December 2019 and May 2020, it accumulated almost two-thirds of all cases of the new coronavirus in China. At the end of April, when the pandemic hit hard in Europe, in Wuhan they assured that they had the infection under control. Now, a team of researchers from Wuhan University has concluded that thousands of Wuhan residents were later infected, although these were asymptomatic cases. The study, which examined more than 60,000 healthy people in China, is published this Thursday in the journal « PLOS Neglected Tropical Diseases».
In Wuhan, the number of Covid-19 cases peaked in February 2020 and the city was initially declared free of the disease in late April, although small clusters of cases appeared in later months. In this new work, Xue-jie Yu, from Wuhan University, China, and her colleagues studied the prevalence of anticuerpos IgG e IgM in blood samples from 63,107 people in China collected between March 6 and May 3, 2020. All were healthy and underwent screening before returning to work.
Rapid antibody tests are used to diagnose current and past infections with the SARS-CoV-2 virus that causes Covid-19. Positive IgG antibodies suggest a previous infection, while IgM antibodies indicate a current or recent infection. Detection of both types of antibodies can provide a better understanding of the number of asymptomatic cases in a population over time.
The percentage of people with positive antibodies against SARS-CoV-2 detected in Wuhan was approximately 1.68%, a figure significantly higher than in other regions of China, where antibody positivity averaged 0.38%. In addition, the positive IgM rate in the city was 0.46%, so the researchers estimate that Thousands of people in Wuhan were infected asymptomatically between March and May 2020, when clinical cases of Covid-19 were no longer reported.
“We concluded that there were a large number of asymptomatic carriers of SARS-CoV-2 after the elimination of clinical cases of Covid-19 in the city of Wuhan, “warn the researchers, whose hypothesis is that the strains that cause asymptomatic infections in the city of Wuhan and other places in China are attenuated viral strains. “Avirulent strains of SARS-CoV-2 can still cause symptoms in extremely susceptible individuals and can also revert to a highly virulent strain to rekindle the Covid-19 epidemic in China,” they note.
For virologist Estanislao Nistal Villán, professor of Microbiology at the School of Pharmacy of CEU San Pablo University, the study has several interesting readings. On the one hand, seroprevalence «tells us about levels of people who have had the virus a lot higher than the figures collected. In the study they talk about 168,000 in Wuhan city alone». It should be remembered that China has officially reported more than 87,000 cases throughout the country since the start of the pandemic. On the other hand, although IgM can be an indicator of a recent infection, “it does not necessarily have to be an active infection, the people diagnosed may have passed the infection days, or even weeks before,” says the virologist.
Regarding the existence of new less virulent ‘strains’, Professor Nistal points out that “in the article it is not proven that there are other circulating attenuated strains that could give rise to seroconversion (appearance of antibodies) and be a reservoir of the virus and that could potentially give rise to infections in immunocompromised patients”. In his opinion, the seroconversion in this case “can be explained due to cases of asymptomatic, or people who did not report, or were not reported, being infected.”
In this sense, Nistal recalls that SARS-CoV-2 does not present as high a virulence as the first SARS and that is one of the factors that have played in favor of the virus and have allowed it to spread further without being detected, as there are more asymptomatic patients. For this reason, he considers it important not to relax the containment measures yet when clinical cases decrease. This summer we already saw how, despite having a very low number of cases, excessive relaxation could allow the virus to recover. Not even with the arrival of vaccines it will be possible to relax at first. “Severe cases and deaths are going to be reduced, and transmission is probably going to be reduced little by little, but the virus will continue to circulate and it will take time to be controlled. We must continue to improve treatment and prevention, “he warns.