The key takeaway for policymakers is how narrow the window for action can be before an outbreak spirals out of control, the researchers say.
The paper estimates that once cases rise beyond roughly two to 10, the disease is likely to spread beyond primary and secondary contacts.
Primary contacts are people who have had direct, close contact with an infected person, such as household members, caregivers or close colleagues. Secondary contacts are those who have not met the infected person but have been in close contact with a primary contact.
If households of primary contacts are quarantined when just two cases are detected, the outbreak can almost certainly be contained, the research found.
But by the time 10 cases are identified, it is overwhelmingly likely that the infection has already spread into the wider population, making its trajectory virtually indistinguishable from a scenario with no early intervention.
To keep the study grounded in real-world conditions, the researchers chose a model of a single village in Namakkal district, Tamil Nadu – the heart of India’s poultry belt.
Namakkal is home to more than 1,600 poultry farms and some 70 million chickens; it produces over 60 million eggs a day.
A village of 9,667 residents was generated using a synthetic community – households, workplaces, market spaces – and seeded with infected birds to mimic real-life exposure. (A synthetic community is an artificial, computer-generated population that mimics the characteristics and behaviours of a real population.)
In the simulation, the virus starts at one workplace – a mid-sized farm or wet market – spreads first to people there (primary contacts), and then moves outward to others (seconday contacts) they interact with through homes, schools and other workplaces. Homes, schools and workplaces formed a fixed network.
By tracking primary and secondary infections, the researchers estimated key transmission metrics, including the basic reproductive number, R0 – which measures how many people, on average, one infected person passes the virus on to. In the absence of a real-world pandemic, the researchers instead modelled a range of plausible transmission speeds.
Then they tested what happens when different interventions – culling birds, quarantining close contacts and targeted vaccination – kicked in.
The results were blunt.
Culling of birds works – but only if done before the virus infects a human.
If a spillover does occur, timing becomes everything, the researchers found.
Isolating infected people and quarantining households can stop the virus at the secondary stage. But once tertiary infections appear – friends of friends, or contacts of contacts – the outbreak slips out of control unless authorities impose much tougher measures, including lockdowns.
Targeted vaccination helps by raising the threshold at which the virus can sustain itself, though it does little to change the immediate risk within households.