No New Swine Flu Strain, So Why Are Cases Rising? Experts Explain the Immunity Cycle Behind H1N1 Surge

New Delhi: Swine flu cases are once again rising across several parts of India, including Delhi. Karnataka has reported more than 4,000 cases, while Delhi has crossed the 1,700-mark. The sharp increase has raised concerns over whether a new strain of H1N1, commonly known as swine flu, is behind the surge.

However, the Indian Council of Medical Research (ICMR) has clarified that no new H1N1 strain has been detected in the country. The infections are being caused by the existing strain.

This raises an important question: If there is no new strain, why are cases increasing again? Experts point to a combination of changing weather patterns, increased transmission in crowded spaces and, importantly, declining population-level immunity.

H1N1 Has Returned in Cycles
India first reported a swine flu case in May 2009. A major rise in infections followed during the winter months from December 2009 to March 2010. Cases subsequently declined, but the virus continued to circulate.

Another significant outbreak occurred during 2014–15, followed by another major increase in cases in 2019. The latest rise is therefore not unprecedented.

A Professor and Director of Community Medicine at Safdarjung Hospital explained that H1N1 has never completely disappeared from the population. Instead, infections tend to rise periodically when conditions become favourable for transmission.

How Does Immunity Affect the Rise in Cases?
According to Dr Kishore, when a large number of people contract a virus, they develop immunity against it. This helps keep transmission under control for several years. Sporadic infections may continue, but widespread outbreaks become less likely.

However, immunity within the population can gradually decline over time. As the number of people with effective immunity falls, more individuals become susceptible to infection, allowing the virus to spread more easily once again.

In simple terms, the virus does not necessarily need to change dramatically for cases to rise. A decline in population-level immunity can create enough susceptible people for infections to increase.

Changing Weather Can Also Fuel Transmission
Weather conditions are another important factor. Flu infections can increase during periods of fluctuating temperatures, rain and humidity.

People also tend to spend more time in crowded or enclosed environments such as offices, schools, public transport and other indoor spaces. Influenza viruses can spread through respiratory droplets and particles released when an infected person coughs, sneezes or talks.

When Will Cases Start Falling?
There is no fixed date for when the current surge will end. According to the expert, transmission generally begins to slow when enough people develop immunity and the pool of susceptible individuals decreases.

The doctor also stressed that there is no need for widespread panic. Most H1N1 infections cause mild illness, although some people can develop serious complications and may require medical attention.

People experiencing flu-like symptoms should consult a doctor, particularly if symptoms are severe or worsening. Annual influenza vaccination can also help reduce the risk of influenza illness and its complications, especially among people at higher risk.

The current rise in cases, experts say, should therefore be viewed as part of the recurring pattern of seasonal influenza transmission rather than evidence of a newly emerged H1N1 strain.