What is immunity debt and why is it causing a spike in swine flu cases

Delhi had 15 patients in August; 1,500 cases & 43 deaths reported from Maharashtra this year

By Taran Deol
Published: Wednesday 24 August 2022
The COVID-19 pandemic has caused people to become more sensitised and willing to get tested for other respiratory illnesses, like swine flu. Photo: iStock

India seems to be at the tapering end of yet another surge of COVID-19 cases, possibly caused by the BA.5 and BA.2.75 sub-variants of Omicron. But the spike was accompanied by an increase in swine flu cases this time, particularly in Delhi and Maharashtra.

The national capital has recorded at least 15 cases through August 2022, while nearly 1,500 cases and 43 deaths have been reported in Maharashtra this year, according to media reports. 

Lockdown measures and social distancing during the COVID-19 pandemic meant fewer swine flu cases in 2020 and 2021. However, hospitals have reported increased number of cases in the outpatient department and those requiring hospitalisation.

Swine flu symptoms are similar to COVID-19, like cough, running nose and sore throat. But is there anything unusual about this surge? 

Read more: When COVID-19 or flu viruses kill, they often have an accomplice — bacterial infections

Viruses usually spread rapidly during the monsoon. But there are two key reasons behind the increase in cases, according to experts. A phenomenon called ‘immunity debt’ and increased sensitisation to healthcare triggered by the pandemic might be behind this. 

Immunity debt is the lack of exposure to common viruses and bacteria, said public health analyst and epidemiologist Dr Chandrakant Lahariya. “More cases of viral illnesses are expected and reported this year because people were either at home or wearing masks for the last two years.”

This has resulted in low transmission of common pathogens that typically build our immune system, said Lahariya. Now that there is a return to routine activity, the population does not have immunity due to less exposure. “People have become more susceptible to infection. This phenomenon is also happening around the world,” he said. 

Several other countries have recorded an unseasonal uptick in cases of respiratory illnesses. For instance, Australia recorded severe respiratory syncytial virus (RSV) cases in late 2020 and the summer of 2021 as pandemic-associated restrictions were eased. 

“Before 2020, RSV activity consistently began during mid-autumn (April–May) and normally persisted for six months with an epidemic peak in the middle of the Australian winter (middle of July, weeks 27–29),” a May 2022 study published in Nature noted. In contrast, RSV activity in 2020 occurred six to nine months later than historically observed.

Read more: African swine fever: Kerala to buy all healthy pigs to help farmers after Wayanad outbreak

Laboratory-confirmed positivity rates were considerably higher than those of the previous three seasons at the peak of RSV activity across each state and territory in Australia, according to the study. The flu season peaked in March after a decade in the United States — another example of what immunity debt can cause. 

But is the rise in swine flu cases in India an actual surge or a function of better testing mechanisms? The surge may be false, reasoned Dr Lancelot Pinto, a pulmonologist at Mumbai’s Hinduja hospital. The COVID-19 pandemic has sensitised people about respiratory illnesses and they are more willing to test. 

“Earlier, swabs were done at the hospital level, but now it’s more accessible with testing kits being developed,” the doctor said. “All these could be reasons behind a potential false surge in cases.” Experts believe the surge in cases seems to result from a combined factor of health system readiness and people voluntarily coming up. 

With swine flu cases rising and COVID-19 yet to make its way out, should we be worried about simultaneously testing positive for both? Dr Lahariya argued that co-infection is not a terminology typically used for common respiratory viruses since it does not have public health or clinical relevance. 

While Dr Pinto has seen some cases of co-infection, it hasn’t resulted in a more severe infection. “This is difficult to interpret because sometimes RT-PCR tests can remain positive for up to a month. So does that make it a co-infection?” he said. 

Read more: COVID-19 under control, Kerala's robust healthcare system now faces outbreak of tomato flu, other pathogens

Swine flu is a human respiratory infection caused by an influenza strain that first affected pigs, thus giving it the name. It was declared a pandemic by the World Health Organisation (WHO) in 2009 when it first infected humans. Following early outbreak reports from North America in April 2009, the virus quickly spread worldwide; by June, 74 countries and territories had confirmed infections.

“Unlike typical seasonal flu patterns, the new virus caused high levels of summer infections in the northern hemisphere and then even higher levels of activity during cooler months. The new virus also led to patterns of death and illness not normally seen in influenza infections,” the WHO noted. 

Today, it continues to circulate seasonally and is included in the influenza vaccine updated annually.

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