Seventy-two people in Maharashtra have died from swine flu in the past three months. The viral infection, which is caused by the H1N1 virus, is seeing a resurgence in western India this year, with Maharashtra alone recording 1,870 cases till August 23.

The caseload across India is likely to have crossed at least 2,800 so far this year, a threefold jump from the previous year. In contrast, H1N1 deaths for the entire country in 2021 stood at 12.

Does the resurgence of the H1N1 virus have anything to do with Covid-19 on the verge of turning endemic or is the virus following a cyclic pattern? spoke to doctors and scientists to understand why H1N1 cases plunged during the past two years only to bounce back this year.

A cyclic pattern

H1N1 is a viral infection with symptoms involving cold, fever, sore throat, body ache and headache. In severe cases, it leads to pneumonia and acute respiratory distress syndrome.

The H1N1 pandemic first broke out in April 2009 and, unlike Covid-19, subsided quickly by 2010. The World Health Organization had declared the end of the H1N1 influenza pandemic by August 2010.

Since then it has been endemic, occurring in the form of a seasonal flu. Epidemiologist Dr Jayaprakash Muliyil, chairperson of the Scientific Advisory Committee at the National Institute of Epidemiology, said that once 40% of the population had attained herd immunity in the 2009-’10 outbreak, the virus transmission slowed down.

“Note that H1N1 is not as transmissible as coronavirus,” he said. “Each virus has a different threshold.” According to Muliyil, even though 60% of the population was unexposed, the number of H1N1 cases began rapidly declining.

Swine flu made a reappearance in 2012, with a high number of cases in Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh and Rajasthan. “Because the susceptibility to infection increased in the population over a period of time,” he said.

Since then, H1N1 has maintained a cyclic pattern. Cases spike around monsoon and winter, with the monsoons recording a larger share of cases than winters in India. “Its susceptibility builds up over a year or two and then cases rise in the following year,” Muliyil said, explaining that herd immunity against the virus wanes.

Before vaccination was rolled out, the measles epidemic too occurred once in two to three years, and rubella once in six to seven years. While a vaccine against influenza to prevent H1N1 was introduced in late 2009, its uptake in India remains low.

In the last decade, H1N1 cases surged in 2015 then 2017 and then in 2019. Since the past two years, however, the number of H1N1 cases have been at an all-time low. Cases in India dropped from 28,798 in 2019 to 2,752 with 44 deaths in 2020. In 2021, there was a further decline to 778 cases and 12 deaths.

But this year, cases are rising again. Data provided by the National Centre for Disease Control for H1N1 till June 30 showed that India recorded 424 cases and 12 deaths in the first half of this year. By July-end, the figure rose to 1,455 cases. At least 17 deaths were recorded in July alone. Trends indicate these figures are expected to rise further in August.

Gujarat and Karnataka are also seeing a rising trend in cases. In 2021, Karnataka had recorded 13 cases but as of July-end, it had 283 cases of H1N1. Gujarat had recorded 33 cases in 2021 and by July-end this year it had 205 cases.

The Pandemrix vaccine against swine flu. Credit: Grook Da Oger, CC BY-SA 3.0, via Wikimedia Commons.

Covid-19 effect?

Some medical experts say the lull in H1N1 over the past two years may have been related to the outbreak of the Covid-19 pandemic in early 2020. For one, Covid-19 appears to have diverted testing resources, which may have led to an underdetection of swine flu cases.

But some medical experts believe Covid-19 may have contributed to low H1N1 circulation as well. Dr Rajesh Karyakarte, Professor and Head of Microbiology Department at BJ Government Medical College, said whenever Covid-19 waves rose, other viruses “sort of disappeared”.

“We saw this with all the three waves,” he said. “Now that Covid-19 is ebbing, H1N1 rise is not surprising.” According to Karyakarte, when a virus infects a person, the body releases interferons. These are signalling proteins released by the body’s immune system to fight infection and other diseases.

Karyakarte noted that Covid-19, due to its high transmissibility, infected almost the entire population. “When an H1N1 virus tried to infect the same person, the interferons would respond making it difficult for H1N1 to cause infection,” he said.

Dr Pradeep Awate, Maharashtra state epidemiologist, also said that coronavirus managed to wipe out other viruses during the peak of the pandemic.

But Muliyil and Dr Gagandeep Kang, professor at the Christian Medical College, Vellore, said there is no scientific evidence to suggest that a dominant virus drives away other viruses.

According to Muliyil, what may have happened is that when two viral infections occur simultaneously in a body, one virus may modify the clinical outcome, duration of infection and severity caused by the other virus. “That may have been the fate of H1N1 infection these past two years,” he said.

Kang said that in the same family of a virus, a dominant variant may replace a weaker one, as seen with the Omicron variant of Covid-19. But there is no evidence to suggest that the coronavirus replaced H1N1 in 2020 and 2021. “Influenza is not as transmissible as Covid-19. Mask adherence may have deterred its transmission further,” she said.

‘Apex of the pyramid’

An analysis of the 72 deaths in Maharashtra showed that 85% of the deceased were 40 years of age or older while 65% had another illness or suffered from a comorbidity.

“In 30 to 35% deaths, oseltamivir medicine was started for the patient within two days of symptoms,” Awate said. Oseltamivir is an antiviral used to treat H1N1. Awate added that the high fatality cannot be attributed to only the delay in treatment.

In Maharashtra, the current case fatality rate, or number of people dying out of those diagnosed, stands at 3.8%. Awate said that not all symptomatic cases are being tested, hence the base pool to calculate fatality rate is small.

There are four categories of patients – those under “A” and “B” have mild symptoms such as a cold and low-grade fever that can be managed at home. Doctors and hospitals may or may not recommend a test for them.

In category “C”, patients develop pneumonia or have lung involvement leading to breathlessness while in category “D”, the symptoms are severe. “Category C and D require hospitalisation and they are being actively tested,” Awate said.

There are also few government laboratories in India that are testing H1N1 as of now. Some laboratories have raised concerns about the lack of sufficient testing kits. At private facilities, the cost per test is between Rs 4,000-Rs 5,000. In Maharashtra, senior health officials have urged the state government to cap the rates of H1N1 testing. The state government is yet to announce a price cap.

Kang, from the Christian Medical College, said 72 deaths in just three months in one state is a high figure. “This is just the apex of the pyramid,” she said. “Essentially, it is telling you there is a lot of virus circulating.”

Globally, the death rate of H1N1 is less than 1%. A higher fatality rate in India indicates a lack of adequate diagnosis. Kang said that compared to the issue of not diagnosing all H1N1 cases, a bigger concern is the complete lack of diagnosis of other influenza-like illnesses. “It is like looking under a lamppost where there is light,” said Kang. “There are other influenza [viruses] which are in the dark and not even getting detected.”

This reporting was supported by a grant from the Thakur Family Foundation. Thakur Family Foundation has not exercised any editorial control over the contents of this article.