Indian Research Reveals Potential H5N1 Human Transmission Risks

Concerns about the H5N1 bird flu, commonly known as avian influenza, have persisted for years as scientists warn of its potential to infect humans and trigger a global health crisis. The virus has been prevalent across South and South-East Asia and has sporadically infected humans since its emergence in China in the late 1990s. According to the World Health Organization (WHO), from 2003 to August 2025, there have been 990 reported human cases of H5N1 across 25 nations, resulting in 475 deaths and a fatality rate of approximately 48%.

In the United States, the H5N1 virus has affected over 180 million birds, infiltrated more than 1,000 dairy herds across 18 states, and resulted in at least 70 human infections, predominantly among farmworkers. This has led to several hospitalisations and one reported death. In January, three tigers and a leopard at a wildlife rescue centre in Nagpur, India, succumbed to the virus, which primarily affects birds.

Human symptoms of H5N1 infection resemble a severe flu, including high fever, cough, sore throat, muscle aches, and occasionally conjunctivitis. While the risk of transmission to humans remains low, health authorities are closely monitoring the virus for any changes that could increase its spread.

Recent modelling conducted by researchers Philip Cherian and Gautam Menon from Ashoka University in India seeks to simulate the potential progression of an H5N1 outbreak among humans and identify effective early interventions. Their findings, published in the BMC Public Health journal, utilise real-world data and computer simulations to explore how an outbreak may unfold.

"The threat of an H5N1 pandemic in humans is a genuine one, but we can hope to forestall it through better surveillance and a more nimble public-health response," Professor Menon stated in an interview.

The researchers suggest that a pandemic could begin quietly with a single infected bird transmitting the virus to a human, typically a farmer or market worker. The primary danger arises not from this initial infection, but rather from the possibility of sustained human-to-human transmission.

To model the potential spread of the virus, the research team used BharatSim, an open-source simulation platform initially designed for Covid-19 modelling, which allows for the study of other diseases as well. Their analysis highlights the critical time window for intervention before an outbreak escalates beyond control.

The study indicates that if close contacts of an infected person are quarantined when only two cases are detected, containment is highly probable. However, once the number of cases rises to ten, the likelihood that the virus has already spread into the wider community increases significantly, making effective intervention more difficult.

The researchers created a synthetic community model based on a village in Namakkal district, Tamil Nadu, an area known for its extensive poultry farming. Namakkal is home to over 1,600 poultry farms and produces more than 60 million eggs daily.

In the simulation, the virus initiates its spread at a workplace, such as a poultry farm or market, infecting primary contacts before potentially reaching secondary contacts through various interactions within households, schools, and other workplaces. By tracking these contacts, the researchers assessed key transmission metrics, including the basic reproductive number, R0, which indicates the average number of individuals infected by one infected person.

The study examined the impact of various interventions, such as culling infected birds, quarantining close contacts, and targeted vaccination. Culling has been found effective if conducted before any human infection occurs. However, if a spillover does happen, the timing of interventions becomes crucial. Isolating infected individuals and quarantining households can effectively contain the virus at the secondary transmission stage. Once it reaches tertiary infections, more stringent measures, including lockdowns, are necessary to regain control.

The study also identified an important trade-off concerning quarantine. Implementing it too early can keep families together for extended periods, raising the chances of infected family members transmitting the virus to others in close proximity. Conversely, if quarantine is enacted too late, it becomes less effective in slowing the outbreak.

Researchers acknowledge that the model contains limitations, relying on a single synthetic village with fixed characteristics and daily movement patterns. It does not account for simultaneous outbreaks caused by migratory birds or poultry networks, nor does it factor in behavioural changes in response to known infections.

Seema Lakdawala, a virologist at Emory University in Atlanta, cautions that the simulation assumes a highly efficient transmission of influenza viruses, which may not represent all strains. She notes that research is ongoing into how efficiently different strains spread among humans, suggesting that only a subset of individuals infected with seasonal flu are capable of transmitting the virus effectively.

In the event that H5N1 establishes itself within the human population, Dr Lakdawala warns that it could lead to significant disruption, likely resembling the 2009 swine flu pandemic rather than the Covid-19 pandemic.

"We are better prepared for an influenza pandemic now, as we have licensed antiviral medications effective against H5N1 strains and stockpiles of candidate H5 vaccines ready for rapid deployment," she stated.

However, complacency could be dangerous. If H5N1 becomes entrenched in humans, it may intermix with existing strains, amplifying its public health impact and potentially leading to unpredictable seasonal epidemics.

The Indian researchers assert that their modelling approach can be run in real-time and updated as new data becomes available, providing invaluable insights for public health officials during the critical early stages of an outbreak.

With ongoing refinements, such as improved reporting and recognition of asymptomatic cases, these simulations could offer vital guidance on the most effective actions to take before the opportunity for containment diminishes.

Health authorities continue to monitor the situation closely, maintaining preparedness and response strategies to mitigate the potential risks posed by H5N1.