One of the first Covid hotspots outside Asia shows promising long-term immunity

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At the start of 2020, an era that seems five lives ago, a small Italian town called Vo ‘was battling a COVID-19 crisis.

Italy had confirmed the first case of SARS-CoV-2 transmitted locally outside Asia in February, so health authorities were ready to take action.

The mayor (who was also the city pharmacist) ordered a strict lockdown, and the governor called for general tests for the entire city of 3,270 people. The roads were cordoned off and soldiers were sent to prevent people from leaving.

Now, on the other side of this pandemic, it looks like almost everything has changed. We know the virus can travel through the air in poorly ventilated spaces – not just with handshakes or elevator buttons. We have several effective vaccines, as well as completely new strains that are even more virulent.

But despite how different things are now, the small town of Vo ‘is still helping scientists understand this virus and how our bodies respond to it.

Along with the mass testing campaigns in February and March 2020, scientists again tested the majority of Vo’s population for anti-SARS-CoV-2 antibodies in May 2020. They found that around 3.5% of the population (about 100 people) had been infected at some point in the past.

In November 2020, they followed up again and asked anyone who had tested positive on the antibody or swab test to take another antibody test to see who still had antibodies to the virus.

The team found that 98.8% of people who tested positive in May 2020 were still responding to at least one type of antigen in November, meaning the antibodies persisted for at least six months – more likely closer to nine since. their first appearance. infected.

“We found no evidence that the 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,” he added. principal author, Ilaria Dorigatti of Imperial College London.

“However, our study shows that antibody levels vary, sometimes markedly, depending on the test used. This means that care should be taken when comparing estimates of infection levels in a population obtained in different populations. parts of the world with different tests and at different times. “

As we are well aware by now, only a small fraction of cases are symptomatic – so many townspeople did not know they were infected until they were tested.

The researchers had also tested family members of those infected, so they were able to investigate how many people each infected person was likely to infect.

Interestingly – at least with this variant at the time – their modeling suggested that 79% of transmission in the village was caused by only 20% of infections. If you were infected with Vo ‘at the start of the pandemic, the chance that you would pass an infection on to a family member was only one in four.

The latter was significantly lower than the number of reproductions of three that scientists estimated occurs when no action is taken to stop the spread of the virus, so this was an unexpected result.

However, this is a real situation very early in the pandemic, and what happened with the 3,000 or so inhabitants of Vo ‘is unlikely to fully align with epidemics with different variants and populations.

“The approach used to estimate the probability of transmission of SARS-CoV-2 within the household provides a useful framework for understanding how demographic and epidemiological factors influence transmission within the household”, conclude the team in their new document.

“The recent detection of new, more transmissible and severe SARS-CoV-2 variants in the UK and elsewhere, reinforces the urgency to improve control strategies, including widespread testing and digital contact tracing, to keep the incidence of SARS-CoV-2 at low levels globally. “

Aside from these future issues, it is a relief to see that the people at Vo still contained persistent antibodies after so many months.

The researcher published in Nature Communication.

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