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COVID antibodies last at least nine months after infection, study finds

Researchers from the University of Padua in Italy and Imperial College London in the UK tested over 85 percent of the 3,000 residents of Vo', Italy, in February and March last year for infection with SARS-CoV-2, the virus that causes COVID-19.

July 19, 2021 / 04:35 PM IST
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Antibody levels remain high nine months after infection with SARS-CoV-2, the virus that causes COVID-19, whether symptomatic or asymptomatic, according to a study published on Monday that analysed data from an entire Italian town.

Researchers from the University of Padua in Italy and Imperial College London in the UK tested over 85 percent of the 3,000 residents of Vo', Italy, in February and March last year for infection with SARS-CoV-2, the virus that causes COVID-19.

They then tested them again in May and November 2020 for antibodies against the virus.

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The study, published in the journal Nature Communications, found that 98.8 percent of people infected in February and March showed detectable levels of antibodies in November.

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COVID-19 Vaccine

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How does a vaccine work?

A vaccine works by mimicking a natural infection. A vaccine not only induces immune response to protect people from any future COVID-19 infection, but also helps quickly build herd immunity to put an end to the pandemic. Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making the spread of disease from person to person unlikely. The good news is that SARS-CoV-2 virus has been fairly stable, which increases the viability of a vaccine.

How many types of vaccines are there?

There are broadly four types of vaccine — one, a vaccine based on the whole virus (this could be either inactivated, or an attenuated [weakened] virus vaccine); two, a non-replicating viral vector vaccine that uses a benign virus as vector that carries the antigen of SARS-CoV; three, nucleic-acid vaccines that have genetic material like DNA and RNA of antigens like spike protein given to a person, helping human cells decode genetic material and produce the vaccine; and four, protein subunit vaccine wherein the recombinant proteins of SARS-COV-2 along with an adjuvant (booster) is given as a vaccine.

What does it take to develop a vaccine of this kind?

Vaccine development is a long, complex process. Unlike drugs that are given to people with a diseased, vaccines are given to healthy people and also vulnerable sections such as children, pregnant women and the elderly. So rigorous tests are compulsory. History says that the fastest time it took to develop a vaccine is five years, but it usually takes double or sometimes triple that time.

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The results also show that there was no difference between people who had suffered symptoms of COVID-19 and those that had been symptom-free.

"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,” said study lead author Ilaria Dorigatti, from Imperial College.

"However, our study does shows that antibody levels vary, sometimes markedly, depending on the test used," Dorigatti said.

Antibody levels were tracked using three 'assays' -- tests that detect different types of antibodies which 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 antibody levels increasing in some people, suggesting potential re-infections with the virus, providing a boost to the immune system.

The findings suggests 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.

"The May testing demonstrated that 3.5 percent of the Vo' population had been exposed to the virus, even though not all of these subjects were aware of their exposure given the large fraction of asymptomatic infections,” said Professor Enrico Lavezzo, from the University of Padua.

"However, at the follow-up, which was performed roughly nine months after the outbreak, we found that antibodies were less abundant, so we need to continue to monitor antibody persistence for longer time spans," Lavezzo said.

The researchers also analysed the infection status of household members, to estimate how likely an infected member is to pass on the infection within the household.

They found that there was a probability of about one in four that a person infected with SARS-CoV-2 passes the infection to a family member and that most transmission (79 percent) is caused by 20 percent of infections.

This finding confirms that the majority of infections generate no further infections and a minority of the infections cause a large number of infections, the researchers said.

The large differences in how one infected person may infect others in the population suggests that behavioural factors are key for epidemic control, they said.

Physical distancing, as well as limiting the number of contacts and mask wearing, continue to be important to reduce the risk of transmitting the disease, even in highly vaccinated populations, according to the study.

The dataset, which includes the results of the two mass PCR testing campaigns conducted in February and March, and the antibody survey, also allowed them to tease apart the impact of various control measures.

The study showed that, in the absence of case isolation and short lockdowns, manual contact tracing alone would not have been enough to suppress the epidemic.

Follow our full coverage of the coronavirus pandemic here.
PTI
first published: Jul 19, 2021 04:17 pm

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