Influenza cases peak during monsoon season with a secondary peak during winter
| Photo Credit: CDC/Douglas Jordan
On March 7 and March 8, several newspapers reported a surge in influenza (flu) cases in the Delhi-NCR region, with some mentioning the “spike” as over 54%. The source of this news was a local survey (LocalCircle) of over 13,000 people in the Delhi-NCR region. While the estimate was not based on clinical testing but on common viral fever symptoms, and the press release had only mentioned “viral illnesses (Covid/flu/viral fever)”, newspaper reports attributed the “spike in cases” to influenza.
According to NCDC’s Integrated Disease Surveillance Programme (IDSP) data posted on February 27, 2025, there were 516 influenza cases and six deaths in January this year. The number of flu cases across India for all of 2024 was 20,414 with 347 deaths. It is difficult to say if the cases reported in January this year are higher than the same month last year as IDSP does not provide segregated monthly data. Compared with IDSP, the U.S. CDC provides a weekly influenza surveillance report.
While influenza infection does occur year-round in India, it peaks during monsoon season with a secondary peak during winter. As per a May 2023 Viewpoint in the Indian Journal of Medical Research, influenza causes “substantial disease and deaths” every year in India. Acute respiratory infection (ARI) surveillance between 2016 and 2018 by six centres spread across India found 15.4% cases were influenza, whereas 12.7% of severe acute respiratory infection (SARI) cases were influenza. Estimating the number of flu deaths is a challenge as “routine testing for influenza is not done in clinical settings and influenza is rarely certified as a cause of death”.
By modelling ‘excess’ deaths using death data from the Sample Registration System and virological data from the influenza surveillance network of 10 labs across eight States from 2010-2013, a June 2020 paper estimated a mean of 1,27,092 influenza-associated respiratory and circulatory deaths per year in India. Estimated flu-associated deaths was high among adults over 65 years and children under five years. Yet, according to the May 2018 guidelines for influenza vaccination issued by the Health Ministry, influenza vaccine is only considered “desirable” for people older than 65 years and children below five years.
WHO recommends the influenza strains to be used in the vaccine for the coming flu seasons months in advance based on the strains that are currently circulating. On February 28, 2025, WHO announced the recommendations for influenza vaccine composition for the 2025-2026 northern hemisphere influenza season. The recommendations made months before the next flu season gives the manufacturers sufficient time to make vaccines. Though there are a few ways to make flu vaccines, including cell-based and recombinant vaccines, the most common method is to use chicken eggs to grow the virus strains. Egg-based vaccine production takes months to finish. Serum Institute’s influenza vaccine is egg-based, while Sanofi makes both egg-based and recombinant vaccines.
While in most cases the strains used in influenza vaccines match with the strains that circulate in the following season, at times the match is not perfect leading to reduced vaccine effectiveness. “When vaccines are introduced, they represent strains identified from the previous year’s circulating viruses. While seasonal mismatches between WHO-declared strains and actual circulating strains can occur, no such mismatch has been reported this time. Currently, the circulating strain in north India is expected to align with the WHO-identified strains for the region,” Dr Rajeev Dhere, Senior Scientific Advisor, Serum Institute tells The Hindu.
As of February 15, 2025, for the 2024-25 season, an estimated 57 million doses of flu vaccine were administered to adults in the U.S, and as of March 1, 2025, nearly 47% of children received a flu vaccine. Influenza vaccine uptake in India has been historically low. It has not increased significantly despite some of the worst flu outbreaks in recent years — 2015 recorded the most number of cases (42,592) and 2,990 deaths in India. Though not at the same scale, there have been large flu outbreaks since 2015 — in 2017 there were 38,811 cases and 2,270 deaths, 28,798 cases and 1,218 deaths in 2019, and 20,414 cases and 347 deaths in 2024. Influenza vaccine is not part of India’s universal immunisation programme.
As per the Health Ministry guidelines, healthcare workers “should be vaccinated”. The vaccine is “recommended” for pregnant women, and children and adults with chronic illnesses, while it is “desirable” for adults older than 65 years and children under five years despite the high mortality in the two extreme age groups. A 2022 study found that only 1.5% of adults aged 45 years or older were ever vaccinated for influenza.
“Influenza vaccine uptake in India remains low (less than 5%). While the market has shown significant year-on-year growth, overall coverage remains inadequate due to low public awareness and the absence of widespread programmes. We have observed an increased uptake of flu vaccines in India with the market growing by 21% in 2025 vs 2024,” says a Sanofi spokesperson. Serum manufactures 3,00,000-4,00,000 doses and can scale up to over a million depending on the demand, Dr. Dhere says.
Published – March 23, 2025 12:00 am IST