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The H1N1 virus driving Delhi-NCR’s current flu surge appears to be changing its structure — a process that is making it spread faster and slip past the immunity built by past infections or vaccines, Dr Randeep Guleria, one of the country’s leading pulmonologists and former director of AIIMS, New Delhi, told .

On Friday, Union health minister JP Nadda reviewed Delhi’s swine flu surge with state officials, as the city’s tally for the year touched 1,777 confirmed H1N1 cases – at least seven times more than last year, same period.

Explaining the science behind the outbreak, Dr Guleria said the virus could be undergoing an “antigenic drift" — essentially, small but constant changes in the virus’s outer structure over time, the same way a criminal might keep changing disguises to avoid recognition. “The virus undergoes what we call an antigenic drift, that it keeps changing with time," he said.

He explained that this drift can have two consequences. First, the altered virus “may become more infectious, it may spread more than what the previous strain was doing."

Second, because the virus looks slightly different from before, the body’s immune system — trained to recognise the older version, whether through prior infection or vaccination — may not fight it off as effectively. “It may change a little bit that may bring down the people’s immunity…," he said.

In simple terms: even people who were vaccinated or previously infected are not fully protected this season, because the virus has “changed its face" enough to partially dodge the body’s defences — which is why more people are falling sick despite past exposure or vaccination.

Weather has added fuel to this mutation-driven spread. Guleria pointed out that monsoon conditions help the virus survive longer outside the body. “Because of the weather, more rain, monsoon, humidity, which allows the virus to sort of survive in the atmosphere for a longer time, we are seeing a surge in the number of cases," he said.

This combination — a mutated, more infectious virus and monsoon conditions that help it linger in the air — is why Delhi-NCR has seen a seven-fold jump in H1N1 cases compared to the same period last year.

For Delhi-NCR residents, Guleria struck a reassuring note while explaining why this year’s wave has felt worse. “We need not panic. We see a surge of flu every year during the monsoon months and also during the winter months. This year, it’s been unusually high because the virus continues to mutate, it has what we call an antigenic drift, and this time, I think that the virus is more infectious," he said.

He added that following basic infection-control measures can help protect individuals and slow the spread community-wide. “As the monsoon wanes, the number of cases will gradually come down," he said.

Several doctors earlier told that they are increasingly seeing patients arrive with breathing difficulty, and some developing pneumonia. Guleria – now Chairman of the Institute of Internal Medicine and Respiratory & Sleep Medicine at Medanta Gurugram – confirmed this trend at his own hospital.

“So, we are seeing a lot of patients. We are admitting patients also with pneumonia…," he said. He added that while most patients recover with care at home or in general wards, “people who are the older age group, people with comorbidities, sometimes have more severe illness, and they really need to be admitted in the ward or in the ICU."

According to Guleria, most people with H1N1 will only experience mild illness — fever, cough, body aches, sore throat, a running or blocked nose, and sometimes stomach upset, nausea, vomiting or diarrhoea. But he flagged specific signs that indicate the infection has worsened and needs urgent medical attention: “If you have chest pain or chest tightness, if you start getting breathlessness, if your cough is increasing and there is phlegm in the sputum, or if you find that your fever is still continuing to be high grade even after four to five days."

For children and the elderly, the warning signs can look different and are easy to miss, he cautioned. Drowsiness, reduced alertness, a child refusing to feed, or visibly faster breathing all indicate the infection has moved beyond the throat and nose into the lungs. “All these features if they are there, these mean that this viral infection, which is usually in the upper airways, that is the throat, nose, has now travelled down into the lower airways into the lung and maybe causing a pneumonia, which may lead to decrease in your oxygen saturation," he said.

A pattern doctors are noticing repeatedly: when one family member is hospitalised, others at home soon develop symptoms too — a sign of just how easily this year’s strain spreads. Dr Guleria explained that the virus can spread even before a person knows they are sick. “This virus spreads—most likely it has become more infectious, it spreads much more, and you will have people in the family who will get it… so the illness becomes infectious one day before you develop symptoms," he said.

He described the typical chain of transmission inside households: “Usually what happens is one person, usually it could be the child who’s bought the infection from the school, brings it to home, he has fever, cough, and cold, then other members of the family also get the infection, and then the whole family suffers from a flu-like illness."

He attributed this partly to lapses in basic hygiene at home, noting that such clusters occur “because we are not following the classical infection control practices at home to prevent the spread of infection."

Keeping sick children home from school, he said, is one of the most effective ways to stop the infection from spreading further. “It’s good not to send the child to school. Because if you send the child to school, then that child will pass on that infection to other class fellows, and they will then take it to their homes, and that’s how the infection keeps spreading," he said.

Guleria said the spread can be significantly curbed with basic precautions. “If you have fever, cough, sneezing, then avoid meeting especially the older age group or children…cover your mouth while coughing and follow cough etiquettes so that you don’t spread the virus in the air," he said, adding that regular use of hand sanitiser is important too.

He also advised masking up in crowded public spaces. “If you are going out and there’s a crowded area, like you are travelling on the metro or in a crowd, wear a mask so that you can get protection against getting the infection," he said, while also urging people with chronic conditions like diabetes or hypertension to stay consistent with their regular medication.

With India’s adult flu vaccination rate below 2 per cent, Guleria made a strong case for immunisation, especially for older adults and those with existing health conditions. “We have been promoting adult vaccination, which includes flu vaccination and pneumococcal vaccination, very aggressively, especially for people over the age of 50 and those who have co-morbid conditions, because they can have a more severe illness," he said.

He clarified that the vaccine will not necessarily prevent infection altogether, but significantly reduces its severity. “I am not saying that you will not get the illness, but you’ll get a less severe illness, and your chances of being hospitalised or having severe symptoms comes down significantly," he said, urging everyone — particularly high-risk groups — to get vaccinated.

The H1N1 virus undergoes constant small changes in its outer structure over time, a process known as "antigenic drift". This means the virus keeps changing with time.

Delhi-NCR residents should get the annual flu vaccine, especially those over 50 or with existing health conditions, as it is the best protection against severe illness. Other precautions include covering the mouth and nose when coughing or sneezing, washing hands thoroughly, using hand sanitiser, and staying home until fever-free for at least 24 hours without medication. It is also advised to prevent stagnant water, use mosquito repellents, and maintain a clean environment.

The H1N1 virus is undergoing an "antigenic drift," which involves small, constant changes in its outer structure over time, allowing it to spread faster and evade immunity from past infections or vaccines. The current reporting does not specify if a new vaccine could be developed for this mutated strain.