In early 2020, a time that feels like five lifetimes ago, a small Italian town called Vo’ was battling a COVID-19 crisis.
Italy had confirmed the first locally transmitted case of SARS-CoV-2 outside of Asia in February, so health authorities were ready to jump into action.
The mayor (who was also the town’s pharmacist) ordered a tough lockdown and the governor requested blanket testing for the whole town of 3,270 people. Roads were sealed off and soldiers were sent to stop people leaving.
Now, on what feels like the other side of this pandemic, it seems like nearly everything has changed. We know that the virus can travel through the air in poorly ventilated spaces – not just with handshakes or elevator buttons. We have multiple effective vaccines, along with completely new strains that are even more virulent.
But despite how different things are now, the little town of Vo’ is still helping scientists understand this virus and how our bodies respond to it.
Along with mass testing campaigns in February and March of 2020, scientists tested the majority of Vo’s population for SARS-CoV-2 antibodies again in May 2020. They found that around 3.5 percent of the population (around 100 people) had been infected at some point in the past.
In November 2020 they followed up again, and they asked all those who had tested positive to either the antibody or swab test to get another antibody test to see who still had antibodies to the virus.
The team found that 98.8 percent of the people who had tested positive in May 2020 still reacted to at least one type of antigen in November, meaning that antibodies were sticking around for at least six months – more likely closer to nine from when they were first infected.
“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 the severity of the infection,” the lead author, Ilaria Dorigatti from the Imperial College London.
“However, our study does show 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.”
As we’re well aware by now, only a small fraction of cases are symptomatic – so many of the town’s people didn’t know they were infected until they were tested.
The researchers had also tested family members of those who were infected, so they were able to investigate how many people each infected person was likely to infect.
Interestingly enough – at least with that variant at that time – their modelling suggested that 79 percent of transmission in the village was caused by only 20 percent of infections. If you were infected in Vo’ early on in the pandemic, the probability that you’d pass on an infection to a family member was only one in four.
This was significantly lower than the reproduction number of three that scientists estimated occurs when no actions are taken to stop the virus’s spread, so it was an unexpected result.
However, this is a real-life situation very early in the pandemic, and it’s unlikely that what happened with the 3,000 or so townsfolk of Vo’ would fit perfectly with outbreaks featuring different variants and different populations.
“The approach used to estimate the probability of within-household SARS-CoV-2 transmission provides a useful framework to understand how demographical and epidemiological factors influence transmission within the household setting,” the team conclude in their new paper.
“The recent detection of new, more transmissible and more severe SARS-CoV-2 variants in the UK and elsewhere, reinforces the urgency of improving control strategies including widespread testing and digital contact tracing, to keep SARS-CoV-2 incidence at low levels globally.”
Those future problems aside, it’s a relief to see Vo’s people still contained lingering antibodies after so many months.
The researcher has been published in Nature Communications.
