London, Nov 18 : Researchers have found an unprecedented resurgence in the invasive meningococcal disease after the easing of control measures imposed during the Covid-19 epidemic, which may see a significant rise in the days ahead. The study, published in the Journal of Infection and Public Health, showed that the recently reported cases have mainly been caused by meningococcal serogroups that were less frequent before the pandemic, and there has been a particular uptick in cases among people aged 16 to 24. Currently, the number of cases is higher than the pre-Covid period. The team from the Institut Pasteur in France predicted that the resurgence could gather momentum in the coming months with the effect of seasonal influenza. The influenza virus creates a favourable context for the development of meningococcal bacteria. All mass gatherings can be a risk factor for infection in general, and especially for invasive meningococcal disease (IMD), they said. "There was an unprecedented resurgence in invasive meningococcal disease in autumn 2022, and now, in autumn 2023, the number of cases is higher than in the pre-Covid-19 period," said Samy Taha, first author and a scientist in the Institut Pasteur's Invasive Bacterial Infections Unit. During the Covid-19 epidemic, health and hygiene measures like wearing masks and social distancing had a positive impact on respiratory infections. This was the case for invasive meningococcal disease (IMD), with the number of infections falling by more than 75 per cent in 2020 and 2021. The researchers expected that either the 'positive effect would last and that meningococci would stop circulating' or 'there would be a rapid resurgence in bacterial activity among a naive population that had not come into contact with the bacteria for a long time". To explore, they conducted a detailed study of the evolution of the disease between 2015 and 2022, and they confirmed the second hypothesis. Compared with a total of 298 cases recorded between January and September 2019, 421 cases have already been recorded between January and September 2023 -- a rise of 36 per cent, even though the winter peak has not yet arrived. The figure for the same period in 2021 was 53 cases. There are two main explanations for this: general immunity was weaker because strains were circulating less, but there was also a decrease in vaccination, with meningitis C vaccination falling by 20 per cent during the first lockdown, for example. So the population has become naive when faced with bacteria that are constantly evolving -- the bacterial genome is highly variable. In other words, the meningococcal bacterial strains responsible for IMD today are different from those that were circulating before the pandemic, and they target different age groups. "It is almost as if the Covid-19 epidemic has reset the entire system," Taha said. /IANS