Summary In February and March 2020, two mass swab testing campaigns were carried out in Vo’, Italy. In May 2020, we tested 86% of the Vo’ population with three immunoassays detecting antibodies against spike and nucleocapsid antigens, a neutralization assay, and polymerase chain reaction (PCR). Subjects who tested positive to PCR in February/March or a serological assay in May were tested again in November. Here we report on the results of the analysis of the May and November surveys. We estimate a seroprevalence of 3.5% (95% credible interval (CrI): 2.8–4.3%) in May. In November, 98.8% (95% confidence interval (CI): 93.7–100.0%) of sera that tested positive in May still reacted against at least one antigen; 18.6% (95% CI: 11.0-28.5%) showed an increase in neutralization or antibody reactivity from May onwards. Analysis of the serological status of members of 1118 households indicates a probability of susceptible-infectious transmission of 26.0% (95% CrI: 17.2–36.9%). Contact tracing had limited impact in suppressing epidemics. |
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Researchers from the University of Padua and Imperial College London tested more than 85 percent of the 3,000 residents of Vo’, Italy, in February/March 2020 for infection with SARS-CoV-2, the virus that causes COVID-19, and analyzed them. again in May and November 2020 for antibodies against the virus.
The team found that 98.8 percent of people infected in February/March showed detectable levels of antibodies in November, and there was no difference between people who had suffered COVID-19 symptoms and those who had no symptoms. The results are published in Nature Communications .
Antibody levels were tracked using three "assays": tests that detect different types of antibodies that respond to different parts of the virus. The results showed that while all antibody types showed some decline between May and November, the rate of decay was different depending on the assay.
The team also found cases of increased antibody levels in some people, suggesting possible reinfections with the virus, which boosts the immune system.
Lead author Dr Ilaria Dorigatti, from the MRC Center for Global Infectious Disease Analysis and the Abdul Latif Jameel Institute for Disease and Emergency Analytics (J-IDEA) at Imperial, said: "We found no evidence that antibody levels between Symptomatic and asymptomatic infections differ significantly, suggesting that the strength of the immune response does not depend on the symptoms and severity of the infection.
"However, our study shows that antibody levels vary, sometimes markedly, depending on the test used. This means that caution is needed when comparing estimates of infection levels in a population obtained in different parts of the world with different tests and At different times . "
Professor Enrico Lavezzo, of the University of Padua, said: "Testing in May showed that 3.5 percent of the Vo’ population had been exposed to the virus, although not all of these subjects were aware of their exposure given the large fraction of asymptomatic infections.
"However, in follow-up, which was performed approximately nine months after the outbreak, we found that antibodies were less abundant, so we need to continue to monitor the persistence of antibodies over longer periods of time."
The team also investigated the infection status of household members to estimate the probability of an infected member transmitting the infection within the household. Their model suggests that there was about a 1 in 4 chance that a person infected with SARS-CoV-2 would transmit the infection to a family member and that the majority of transmission (79 percent) is caused by the 20 percent of infections.
This finding confirms that there are large differences in the number of secondary cases generated by infected people, with the majority of infections generating no further infections and a minority of infections generating a large number of infections.
The large differences in the way an infected person can infect others in the population suggests that behavioral factors are key to controlling the epidemic, and physical distancing, in addition to limiting the number of contacts and the use of masks , remains important to reduce the risk of transmission. the disease, even in highly vaccinated populations.
The team’s data set, which includes the results of the two mass PCR testing campaigns conducted in February and March and the antibody survey conducted in May and then again in November, also allowed them to unravel the impact of various control measures. .
They showed that in the absence of case isolation and short lockdowns, manual contact tracing alone would not have been sufficient to suppress the epidemic.
Project leader Professor Andrea Crisanti, from the Department of Life Sciences at Imperial and the Department of Molecular Medicine at the University of Padua, said: "Our study also shows that manual contact tracing, searching for positive individuals on the database of known and declared contacts - would have had a limited impact on containing the epidemic, if it had not been accompanied by mass screening."
Dr. Dorigatti added: "It is clear that the epidemic is not over, neither in Italy nor abroad. Looking ahead, I believe it is of fundamental importance to continue administering the first and second doses of vaccine, as well as to strengthen surveillance , including contact tracing. Encouraging caution and limiting the risk of acquiring SARS-CoV-2 will continue to be essential."
