H1N1 in India: What the Rising Cases Tell Us

From an epidemiological perspective, how concerning is the recent surge of H1N1 cases in India?

Any year-to-year spike should be of concern. Is this of a specially high concern? I think when you hear there is an “x”-fold increase from 2025, it sounds concerning. But the comparison can be a little misleading because the 2025 baseline against which we are measuring this year’s spike was especially mild in terms of influenza. This is not a novel strain; it has been circulating for some time. And if you take a longer baseline—particularly looking at the pre-2020 period, since the years after 2020 have been somewhat anomalous—the current numbers are not especially concerning. That said, anecdotally, I do know a number of people who have had H1N1 and found the experience quite unpleasant. But, on the whole, this appears to be seasonal influenza activity that is somewhat higher than in recent years.

Delhi, Bengaluru, Mumbai, Pune, Nagpur, and Lucknow have all reported cases. Is this a random geographical pattern? And how robust is the surveillance system in mapping and understanding the spread of the outbreak across different parts of the country?

It’s a little difficult to tell because the surveillance systems differ from State to State, so the variation we are seeing does not necessarily reflect the strength of those systems. Equally strong surveillance systems can still have different ways of describing a case versus what is not a case. What you need is consistency across surveillance systems to make what we call apples-to-apples comparisons. It’s a little difficult to do that in this situation.

The other thing is that, from what I have seen, absolute case counts are being reported: there are “x” number of cases from Kerala, for example. In epidemiology, we map out a denominator. The absolute case count is of limited utility. You need to know “x” number of cases among how many individuals during a particular time frame. Getting an estimate of incidence is what allows you to make those regional comparisons by State or region.

It’s difficult to tell where the highest number of cases per population are occurring, for those reasons.

Could you talk about the importance of vaccines in curbing swine flu, and have we seen equity in terms of the influenza vaccine overall?

Every year, WHO identifies the influenza virus strains and subtypes most likely to circulate during a particular season, which helps guide the composition of the seasonal vaccine. The strains identified for this season included the H1N1 subtype we are currently seeing in circulation, so the vaccine was a good match for the circulating virus.

From year to year, the influenza vaccine’s efficacy ranges from 40-60 per cent—sometimes higher, sometimes lower. Not every vaccinated individual is fully protected because influenza vaccines are always a moving target. At the same time, a vaccine that is 60 per cent effective is still better than one that is not effective at all.

The issue is that only a very small proportion of individuals are being vaccinated. This is a problem in many parts of the world. Uptake of seasonal influenza vaccines is suboptimal. It’s still not too late to get the seasonal flu vaccine if you have not already fallen ill with H1N1. I would advise—if you have the capacity and if your doctor advises—getting the influenza vaccine.

When an infectious disease spreads and the number of cases begins rising, how do epidemiologists determine whether they are seeing a normal seasonal increase or something that could develop into an epidemic?

We use a combination of things. We look at the virulence of circulating pathogens: is it causing more severe outcomes among those who get infected? What most of the evidence points to is that the current circulating strain is not more virulent. Virulence matters because if a higher proportion of infected individuals develop severe disease, it places a greater burden on the health system. That is something we need to monitor closely.

Another thing we look at is the absolute number of cases relative to baseline years. There is a relatively high increase in cases this year relative to last year. But you cannot use only one year as a comparator; you have to use a longer baseline to compare the current year against.

The third thing to keep in mind is the denominator. For instance, “x” number of cases in Uttar Pradesh, the most populous State in India, cannot be directly compared with the same number of cases in a much smaller State without accounting for population size. We also need to consider the surveillance system itself—how cases are identified and whether case definitions are consistent—to make fair comparisons.

Of course, from COVID-19, we all know the other epidemiologic terms we are often concerned about, such as the effective reproduction rate—how many additional cases result from a single case at any given time. If that number is greater than one, we have a growing epidemic. If it is less than one, we have a shrinking epidemic.

But, in general, those are more behind-the-scenes measures. The comparison, the baseline, the virulence, and the incidence of the disease are what matter most.

There is a burden of H1N1 cases going undiagnosed. Given how extensively you have worked in India, do you see any shift since COVID-19—in terms of political will or the surveillance system?

One major shift is that many States across the country strengthened laboratory testing capacity in the public system. That is one of the reasons behind the spike in cases we are seeing. An initial question to ask is: is this really a spike in cases relative to last year, or is this an increase in surveillance? Are more people actually being tested?

One of the metrics we look at is test positivity. A very high level of test positivity indicates that not enough tests are being done. A low test positivity might indicate the opposite. The strengthening of testing facilities has led to more individuals being tested as well.

The other change in India is greater awareness of the role of vaccinations. We typically think of vaccines as childhood interventions, but I have increasingly seen adults understanding and appreciating the value of vaccination. Still, influenza vaccination rates are lower than they should be. Hopefully, that continues to change, with more individuals getting vaccinated for flu.

There are other vaccines for which older adults are eligible, which would protect against common causes of illnesses such as pneumonia. I would like to see that shift continue as well, with more older adults getting vaccinated.

Several positive shifts have occurred as a result of COVID-19: strengthened surveillance systems and an expanded understanding of the value of vaccines, along with systems already set up to deliver them to older adults. Now that these systems are in place, it is time to make sure we are taking advantage of them—getting cases properly diagnosed and vaccinating older individuals.

Sequencing is another area that has been strengthened since COVID-19. The sequencing infrastructure set up around the country in response to COVID-19 is now helping identify the strains circulating in India. That has been another positive outcome of COVID-19 for other diseases like H1N1.

To what extent are recurring outbreaks shaped by environmental and urban conditions?

An outbreak is driven by multiple factors. Viral transmissibility is one; the way we interact with one another is another. One of the things we tried to do in response to COVID-19 was minimise social contact, through lockdowns. That is not always going to be an option for every outbreak—for every flu season, for instance.

What we can do is stay home when we are sick and limit contact with others. It is not just COVID-19 that should keep us from interacting with others while ill—these are all communicable diseases. If you are sick, you should stay home, rest, and follow your doctor’s advice. In urban areas, where people interact more and live in denser conditions, that increases opportunities for certain pathogens to spread. Diseases tend to spread in homes with more people living together, whether in urban or rural areas. I would be hesitant to say we should change the way we live in order to prevent seasonal flu outbreaks. The best thing we can do is get vaccinated and rely on that as a strong prevention intervention for reducing the virus’s impact year to year.

What determines an individual’s vulnerability to severe illness from a respiratory illness such as H1N1, and how much should prevention strategies focus on identifying and reducing the vulnerabilities that make some people more susceptible to severe disease?

Not to sound like a broken record, but the best thing we can do to improve people’s immune systems is to expose them to a denatured component of the pathogen we are concerned with, to build their immunity—that is what we call a vaccine. But on vulnerability and severity of disease, it depends on the pathogen itself.

Sometimes, having a strong immune system can make you more vulnerable to your immune system going into hyperdrive. For other diseases, a weak immune system allows the pathogen to overcome the immune system and cause more serious disease. So it really is pathogen by pathogen. We saw with COVID-19 that conditions such as diabetes, cancer, and hypertension put people at higher risk of severe disease. Those tend, on average, to increase the risk of severe outcomes. But with H1N1 or other diseases, it really depends on the disease itself.

Do diet and exercise matter?

Being fit and healthy is always going to be an advantage. It is always better to follow a healthy diet and be physically fit. Not everybody has that option, and those individuals are usually at higher risk of poor outcomes.

How should we think about the relative importance of institutional measures and individual behaviour in preventing infectious diseases?

Both are important. As a public health person, I am usually thinking about what the public sector is doing to create the conditions in which people can protect themselves or make decisions to best protect themselves and their families. I see the two as linked. My favourite intervention is vaccination, where the government has already established significant infrastructure. For childhood and routine vaccines, the government has done a tremendous job of making them available for free to everyone. Expanding that to other vaccines for other age groups would provide additional protection at a population level.

Even though we are making individual decisions about whether to be vaccinated, my being vaccinated benefits those around me—so we should see vaccines as a public good.

How important is it to involve local communities and frontline health workers in identifying the particular conditions driving these outbreaks and designing strategies to prevent them, especially in the congested cities of India?

Understanding community concerns, understanding why a particular community may or may not accept an intervention, and knowing what can make an intervention more acceptable—these are incredibly important. There is a perception that these things are done through a top-down approach. Of course, health is a State subject. I cannot say that every single health system fails to engage those communities or frontline and higher-level health workers.

You cannot do public health without engaging the public. It is critically important, and we have seen around the world what happens when a strict top-down approach is implemented without engaging the public: you risk fomenting distrust of the health system itself. One of the big lessons for me from COVID-19 is that we need to be better about communicating transparently with the public and engaging the public in developing and designing public-health interventions.

How should we look at the relationship between humans, animals, and the environment, including the conditions in which animals are raised and kept, in order to prevent zoonotic diseases?

The field of public health that deals with human, animal, and environmental interactions is called One Health. It is a growing area of interest for public health practitioners and researchers globally. When poultry, pigs, and other animals are raised for human consumption, in many parts of the world, the conditions they are forced to live in create the conditions for disease to spread rapidly. We often pump them with antibiotics to keep them healthy, which then creates the conditions for antimicrobial resistance to develop. It also creates the conditions for some of these pathogens to jump to humans, as we have seen in a number of instances over recent years. This is one of the reasons we are seeing an increased number of epidemics globally.

These practices, and how we monitor them, are of high concern for people in public health, as are the steps we can take to reduce the risk of antibiotic resistance and of spillover from animal species into humans. Somewhat related is deforestation, where we are pushing further into areas where humans and some of these animals have not previously interacted much—another opportunity for spillover from animals to humans. There is evidence that this is one of the challenges with the Ebola virus outbreak currently under way in sub-Saharan Africa.

One of the reasons I think One Health is such a powerful framework is that it treats human, animal, and environmental health as a common objective to work towards.

What would a genuinely preventive approach to infectious diseases look like? And did the COVID-19 crisis give us any important clues about how such an approach might work?

It is a tall order. There are a number of parallel things we should pursue to address the overall burden of infectious diseases. We know that prevention is often preferred over treatment, so I would always prioritise preventive measures while ensuring we have a system in place to treat those who still fall ill.

India has done a tremendous job of reducing the burden of infectious diseases, largely through vaccination. We have eliminated polio, eradicated smallpox, and driven down child deaths dramatically through vaccination—all because of the power of vaccines and the systems set up to get them into the arms of those who need them. Continuing our focus on vaccination is one major thing we can do.

What happened in the United States and Europe at the beginning of the last century, from 1900 to around the 1950s, was a dramatic decline in infectious-disease mortality—before the widespread availability of vaccines or antibiotics. So what was driving down mortality from infectious diseases across Europe and North America in that period? It was really the establishment of good health authorities at the local level, focused on water, sanitation, living conditions, and hygiene—things that sound basic but can have a huge impact on the spread of infectious diseases. I would say a focus on interventions we know work to stem transmission is what matters.

The other thing we can do is reduce individuals’ vulnerability to infectious disease. Concurrent with those changes in the US and Europe was an improvement in childhood nutrition. When children are better nourished, we see a reduction in mortality associated with infectious diseases as well. Those are the kinds of interventions that work for what we call endemic infectious diseases—childhood diarrhoea, pneumonia, and so on. For novel pathogens—the Ebola virus, or COVID-19 when it first began spreading globally—we need two things in place. First, a better surveillance system to identify when a new or re-emerging pathogen is beginning to cause disease: early-warning systems, which can work through wastewater surveillance and other novel approaches. Second, systems that can respond to novel pathogens when they arise—thinking about how to contain them and how to scale up interventions such as vaccines within a time frame that prevents the largest possible number of cases or deaths.

India, as the global manufacturer of vaccines for the world, has an important role to play in having systems in place that can respond quickly to the emergence of a new pathogen.

How can better urban infrastructure and planning help address public-health challenges, particularly those arising from monsoon flooding, water contamination, and vector-borne diseases?

In a lot of urban areas—something I have been working on more recently—the intersection between water and infectious disease, viewed through a One Health lens, is becoming increasingly important.

With increased monsoon flooding resulting from climate change, groundwater and surface water now mix in ways they had not previously, bringing with them contaminants from surface-water sources. This introduces contaminants and pathogens into water used for the livelihoods of hundreds of thousands, if not millions, of people within these broader water systems. Identifying ways of remediating water contaminants using nature-based solutions can be one way of addressing infectious disease.

Another consequence of increased monsoon flooding is more standing water accumulating in urban areas with more impervious structures that prevent water from dissipating. That is a breeding ground for vector-borne diseases, and we are seeing an increase in that as well.

Infrastructure can be organised to reduce the mixing of groundwater contaminants while using nature-based solutions that are effective and relatively affordable. These need not be expensive infrastructure projects; they can be implemented affordably and even used as an opportunity to give civic spaces to the city. That is especially valuable when communities are engaged as part of the process—which goes back to one of our earlier questions about how we engage communities in these public systems. You can involve them in designing interventions meant to improve the cleanliness of water bodies, create public spaces for recreation, and address public-health concerns at the same time.

Could you weigh in on just how political an epidemic is, because when one hits, the government gets to decide who gets the vaccine, and all the systemic inequalities that come into play?

Public health is political. You do not have to look further back than COVID-19 to see that. Everywhere in the world, COVID-19 was a political issue. If you think of public health as the government playing a central role, politics decides how a government’s resources are allocated—and so the allocation of resources is a political decision. It is not just the controversial things we think of as making public health political, or the epidemic itself, or anything within a health system that draws attention. Even the most seemingly mundane aspects of public health are driven by political decision-making. Public health is very much political.