People who caught and recovered from Covid-19 may be reinfected with a different strain of the pathogen, a new study from South Korea shows. Researchers found that a young woman infected with one strain of the coronavirus was, just weeks after recovering, reinfected with a different strain of the virus. The findings indicate that the patient’s immune response to the first strain did not protect her from the second and highlight how mutations could make controlling Covid-19 more difficult than first thought. South Korea is at the centre of efforts to understand reinfection after reports in April that scores of people who recovered from the virus later went on to test positive for Covid-19 again. Researchers sequenced the genetic material, or RNA, of two types of coronavirus found in an affected 21-year-old woman: one type from when she was initially infected and the other from when she was reinfected. The findings, released in <em>Clinical Infectious Diseases</em>, detailed key differences in the genetic material coding for the spike protein, a structure that sits on the outside of the coronavirus – giving the virus its crown-like appearance. Using computerised analysis, the researchers produced an evolutionary tree for several samples of the virus. They found that the types from the woman were different strains that belonged to different clusters. These clusters are characterised by particular changes or mutations in the genetic material of the pathogen. The first type of coronavirus the woman was infected with belonged to an evolutionary grouping or clade called V, while the strain she was reinfected with was from clade G. “The viral RNA from the positive retest was clustered into a subgroup distinct from that of the initial infection, suggesting that there was a reinfection of Sars-CoV-2 with a subtype that was different from that of the primary strain,” the study's authors wrote. During an infection, the body recognises the spike protein of the coronavirus and this causes the immune system to develop antibodies that destroy the virus particles. In subsequent infections, this immune response should confer protection against the coronavirus. The new study, preliminary results of which were reported by South Korean media in September, showed that the immune response from an initial infection may be ineffective against genetically distinct strains that have changes to the spike protein. “Sars-CoV-2 infection may not confer immunity against a different Sars-CoV-2 strain,” the authors wrote. While there have been several reports of reinfection, it is thought to be rare. The way that some strains of the coronavirus may not be susceptible to the immune response to other strains raises parallels with influenza. This pattern influences which flu vaccine is given to people each year. "The [flu] vaccine composition is reviewed each year and updated as needed based on which influenza viruses are making people sick, the extent to which those viruses are spreading, and how well the previous season's vaccine protects against those viruses," the US Centres for Disease Control said. But a study by Australia’s national science agency conducted on ferrets and published last month concluded that a particular change in the coronavirus spike protein between two distinct strains was unlikely to affect the vaccine effectiveness so would not require regular “vaccine matching”.
