We survived COVID, but getting sick so often now? Here’s what 'immunity debt' means, and why you MUST know it

New Delhi: It’s a mix of coughs and sneezes at the OPD of a major hospital in Delhi. “I had a fever and cough two weeks ago. The fever went away in three days, but the cough stayed,” says a patient waiting to see the doctor.

Another one adds, “Earlier, I rarely fell ill during season change. But after Covid, I catch something every time. I don’t know why.”

As per a TOI report, many people across India are now falling sick more often with coughs, fevers, and other seasonal illnesses. While some believe this could be due to long Covid, others are talking about something called ‘immunity debt’.

What is immunity debt?

The term was first used in 2021 when New Zealand saw a sudden rise in babies being hospitalised with RSV (a virus that causes cold-like symptoms). Experts say ‘immunity debt’ means that when people don’t come into contact with common germs for a long time, their immunity weakens.

According to a recent study by the University of Oxford and Peking University, the Covid safety rules, like staying home, wearing masks, washing hands, protected us from Covid but also stopped us from getting exposed to other viruses. Once the restrictions ended, people started falling sick more easily.

“The stricter the Covid rules in a country, the bigger the flu outbreak afterwards,” study author Li Chen of Peking University told TOI. Oxford expert Daniel Prieto-Alhambra added, “By avoiding flu for a few years, we now have a population that’s more vulnerable to infections.”

The study, published in Advanced Science, looked at flu data from 116 countries between 2012 and 2024, including India. It found that during lockdown, global flu cases fell by nearly 46 per cent. But in 2022, after restrictions eased, flu cases jumped by 132 per cent compared to before the pandemic.

The study warns that this kind of immunity debt could lead to major flu outbreaks in the years after a pandemic.

In India, the lockdown lasted from March to May 2020, with distancing measures continuing much longer.

What do Indian doctors say?

Dr Suranjit Chatterjee, senior internal medicine consultant at Apollo Hospital in Delhi, told TOI that cases of flu and other viral infections have become more severe and frequent after the pandemic. However, he says more research is needed to confirm if this is due to immunity debt.

Dr Rommel Tickoo, internal medicine director at Max Hospital, Delhi, said it could also be linked to long Covid symptoms or post-Covid weakness.

A separate study by analytics firm Airfinity and Bloomberg showed that over 40 countries have seen disease outbreaks since 2020 that were 10 times worse than pre-Covid levels. Measles, once eliminated in the US, has returned.

Whooping cough is rising too. Experts say this might be linked to people skipping vaccinations during the pandemic.

Does the theory really hold?

Dr N K Mehra, immunologist and senior scientist at the Indian Council of Medical Research (ICMR), told TOI that the immunity debt theory goes against the concept of ‘immune memory’.

He explained that once someone catches a virus or gets vaccinated, the body remembers how to fight it. But in the case of flu, this memory doesn’t always work well because the virus keeps changing.

A 2022 study by the University of New South Wales in Australia found that a few people had an overactive immune system months after having Covid, but researchers said such cases were rare and not proof of long-term immune damage.

Still, some experts believe immunity debt could be real. Michael Levin, a professor at Imperial College London, told The Guardian that children and adults build immunity by being exposed to viruses.

“It’s possible that people today have met fewer viruses and are now more vulnerable,” he said. However, he added that scientists still don’t fully understand how much repeated exposure is needed to maintain immunity.

Some illnesses, like measles and smallpox, give lifelong protection after just one infection.

Can vaccines help?

Experts agree that vaccines are the best way to prevent disease surges. Even if immunity debt is real, vaccines can help reduce the risk.

Dr Shuchin Bajaj of Ujala Cygnus Healthcare told TOI that future pandemic plans should balance safety measures with efforts to keep up our immunity, especially against regular seasonal viruses. “It’s better to relax restrictions slowly, give off-season vaccines, and be ready for infection spikes after a pandemic,” he said.