According to the results of a new study called SIREN carried out by Public Health England (PHE), most people who have been infected with COVID-19 are likely to have immunity, i.e. be protected against becoming infected again, for at least five months.

The study regularly tested almost 21,000 healthcare workers from the U.K. between June and November of 2020. Among the participants, 6,614 tested positive for antibodies against COVID-19, while more than 14,000 had no signs of previous infection. However, of the infected individuals, 44 developed possible new COVID-19 infections, which represents an 83% level of protection against subsequent reinfection. Therefore, the study results show that past infection with COVID-19 was linked with an 83% risk reduction of becoming infected again.

Researchers also highlighted the fact that protection against COVID-19 by means of a previous infection was not absolute. This means that some people can become infected again within the five-month period, and it is still unclear how long immunity lasts.

According to the authors of the study, there is still the possibility of individuals who have some immunity to carry the virus in their nose or throat and transmit it to others.

“We now know that most of those who have had the virus, and developed antibodies, are protected from reinfection, but this is not total and we do not yet know how long protection lasts,” said Dr. Susan Hopkins, senior medical adviser at PHE and co-author of the study.

“Even if you believe you already had the disease and are protected, you can be reassured it is highly unlikely you will develop severe infections. But there is still a risk you could acquire an infection and transmit (it) to others,” she said.

The research study will continue to monitor the healthcare workers for 12 more months to see whether protection lasts even longer. According to the authors, the five-month length of protection identified means that people who caught the virus in the first wave of infections may now be able to become infected again. The study will continue to look into the impact of the new COVID-19 variant and the effectiveness of COVID-19 vaccines.

“The SIREN study has major implications for how we can get out of the current crisis. The good news is that this study gives further weight that reinfections of COVID is rare, at least at this stage, and that having antibodies will provide protection for a meaningful amount of time, although it may not be lifelong immunity,” said Dr. Simon Clarke, Associate Professor in Cellular Microbiology, University of Reading, to the Science Media Centre.

The study demonstrates the importance of COVID-19 testing in determining its prevalence in the community and the workplace, as well as in reducing infection rates and the spread of COVID-19.

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