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A new study shows that previous Covid-19 infection protects against reinfection even without antibodies

A new study carried out using existing laboratory test data shows that people who had a positive PCR test for Covid-19 but a negative serology test, meaning that they did not produce detectable antibodies, still had a significant amount of protection from reinfection.

The paper, published in the Journal of Infection, shows that the reinfection rate for this group was 0.89%, compared with a 4.36% infection rate for those who had never had Covid-19 before. In fact, the reinfection rate for those with a positive PCR test but negative antibody test was around the same as those who had produced antibodies following infection.

The doctors and scientists, based in Microbiology at St George’s Hospital, worked with colleagues in South West London Pathology (SWLP), North West London Pathology, Newcastle-upon-Tyne Hospitals NHS Foundation Trust and King’s College Hospital. They looked at 49,450 patient laboratory records from the four organisations. Of these, 2,311 were infected in the first wave, of whom 244 had a positive PCR test but negative antibody test. From this group, only two were reinfected with the virus, which suggests a protection level of around 80%.

This study supports the findings of a research study carried out at St George’s Hospital at the beginning of the year, which showed that patients with a positive PCR test for Covid-19 had around 94% protection from reinfection. However, these results suggest that it is not only the antibodies produced after infection that support a level of protection. Lead author and former SWLP Clinical Director Dr Aodhán Breathnach says, “The findings of this study remind us just how complex and robust our immune systems are. The results also show how much you can learn from existing laboratory datasets to support and inspire more sophisticated research projects.”

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