Flu Infections could reduce Covid-19 deaths, finds study

Could flu infections have given some Indians herd immunity to Covid-19? This is what a study by Bengaluru-based scientists says.

The study, which was conducted by the Indian Institute of Science (IISc), uses disease and vaccination data in the public domain with respect to measles, Hepatitis B virus, polio, tuberculosis and flu from 25 countries across the globe, including India.

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In their study, the authors have revealed: “There is no correlation between Covid-19 incidences or deaths, as well as vaccination coverage, with respect to diseases such as measles, Hepatitis B virus and polio. However, countries with lower cases of tuberculosis and higher cases of flu have a significant correlation with respect to Covid-19 deaths.”

While global studies previously tried to correlate BCG (a tuberculosis vaccine) with the reduced number of deaths due to Covid-19 in certain countries such as India, this study is one of the first to also link flu exposure to reduced Covid-19 fatalities.

The key observation is that countries such as India and Australia which have a high percentage of flu-positive samples, have also been observed to have a lower Covid-19 death rate. In addition, the researchers discovered that in countries such as the United States, Italy, Spain and France where health officials regularly conduct flu vaccinations, Covid-19 induced deaths showed a “significant positive correlation value of 0.42.”

The study, which is under peer-review, postulates three primary reasons for why a group of people previously exposed to the flu would enjoy a lower incidence rate of Covid-19.

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First, the exposure to the flu may be lowering the surface expression of ACE2 receptors on human cells which serves as the entry point for the coronavirus to hook onto and infect the cell. Secondly, non-specific immunity is also increased upon flu infections - as is already known for several other viral infections.

The third reason is viral interference, a phenomenon in which one virus hinders the replication of another virus in the body. Among the evidence cited is a nine-year-study of people with Influenza A infections. Among them, it was discovered that the flu had prevented subsequent rhinovirus infections.

The study determined that a combination of flu exposure, plus a country with a BCG vaccination programme, such as India, Egypt, South Africa, show a relatively lower fatality rate of Covid-19 in that country.