Pandemic from Covid-19: A study from Padua and London focuses on antibody responses and the impact of contact tracing.


How many and how long do antibodies last for those infected with SARS-CoV-2? What is the likelihood of infection from someone within the same family unit? How long can we use contact tracing to contain the epidemic?

A team of researchers from the University of Padua and the Imperial College of London set out to answer these questions in a study published in Nature Communications entitled SARS-CoV-2 antibody dynamics and transmission from community-wide serological testing in the Italian municipality of Vo’. The study collected the results from the serological screening of the Vo’ population (Padua, Italy) to estimate the antibody dynamics in SARS-CoV-2 infections, the likelihood of transmission of the virus within households, and the impact of contact tracing aimed at containing the epidemic.

In February and March of 2020, researchers conducted two mass swab testing campaigns detecting new SARS-CoV-2 coronavirus variants in Vo', Italy. The results showed that a significant proportion of infected individuals were asymptomatic (42.5%). Despite this, early isolation of infected people showed that viral transmission could be effectively and rapidly suppressed (see Suppression of a SARS-CoV-2 outbreak in the Italian municipality of Vo' published in Nature).

After the national lockdown, researchers tested 86% of the Vo' population in May of 2020 (2602 test subjects). Researchers used three different types of immunological tests. Researchers were not only able to detect the presence of antibodies against viral antigens spike (S) and nucleocapsid (N) but to identify neutralizing antibodies. Such antibodies block the SARS-CoV-2 virus, making them unable to infect cells. In November of 2020, researchers retested those who had tested positive in February and March, as well as those who had tested positive for at least one of the immunoassays conducted in May.   

Co-author of the study, Professor Enrico Lavezzo of the Department of Molecular Medicine at The University of Padua explains “Thanks to the results obtained from the various tests we believe that as of May 3.5% of the population had been exposed to the virus. As of November, tests showed a reduction in antibody titers, although 98.8% of subjects had retained a detectable amount of antibodies. On the other hand, 18.6% of the subjects showed a marked increase in the antibody or neutralizing titer between May and November, a probable or documented sign of re-exposure to the virus. In essence, our study shows that antibodies can last up to at least nine months. Also, there is no difference between those who contracted the virus symptomatically or asymptomatically, nor in quantity or duration."

Ilaria Dorigatti of the MRC Center of Global Infectious Analysis at Imperial College London comments by saying "Special thanks go out to the population of Vo'. Their mass participation in serological studies allowed us to understand how antibody levels vary over time and, for the first time, to quantify how much transmission occurs within the family and the impact of contact tracing on the epidemic. This study shows that antibody levels vary, even markedly, based on the antigen and the kind of test used. Meaning, we must be cautious when comparing seroprevalence estimates obtained from different tests, at different times, from different parts of the world.  The work demonstrates that mathematical models are useful tools for reconstructing a coherent view of the evolution of an epidemic and quantifying the impact of the various interventions implemented. Our results suggest that there is roughly a 1 in 4 chance that a SARS-CoV-2 infected person will pass the infection to a family member.  We estimate that in Vo', the epidemic has been suppressed thanks to the isolation of infected cases and a brief lockdown, while contact tracing had little effect on the outbreak. Furthermore, the epidemic is not over, neither in Italy nor abroad. I think it is important to move forward with the administration of the first and second vaccine doses and to monitor transmission. For us to identify variants we must substantially strengthen the genotyping of the virus, and continue tracing of contacts, for example with the digital contact tracing.”

Director of the Department of Molecular Medicine of the University of Padua Andrea Crisanti says, “The study shows that contact tracing activities searching for positive individuals, based on known and declared contacts, would have little impact on the containment of the epidemic (finding 44% of infected individuals), had it not been supported by mass screening. For this reason, we believe that for the control of future SARS-CoV-2 outbreaks it is necessary to implement rigorous testing strategies and improve contact tracing approaches. If the contact tracing methodology was not sufficient in a small community like that of Vo ', which has just over 3000 inhabitants, it is difficult to think that it could be in a medium and large city where social interaction networks are amplified and less traceable.”