Most pandemic viruses jump to humans without prior adaptation: Study

What Happened

A landmark study published in the journal Cell (March 2026) by researchers at the University of California San Diego challenges a foundational assumption in pandemic science: that animal viruses must evolve and adapt in an intermediate host before they can infect and spread among humans.
Analysing viral genomes from major pandemic events — COVID-19 (SARS-CoV-2), 2009 H1N1 influenza, West African Ebola (2014), Angolan Marburg, mpox, and SARS-CoV — the team found "no evidence of special evolutionary adaptation before spilling over" into human populations.
Crucially, measurable changes in natural selection (adaptation to humans) appeared only after sustained human-to-human transmission began — not before the initial spillover event.
One notable exception: the 1977 H1N1 influenza re-emergence showed genetic signatures consistent with laboratory-adapted strains, aligning with historical suspicions that it was accidentally released from a research facility.
The findings have direct implications for pandemic preparedness: surveillance must focus on detecting spillovers as they happen, not on waiting for "pre-adapted" viruses to emerge.

Static Topic Bridges

Zoonotic Spillover: How Animal Viruses Jump to Humans

Zoonosis refers to an infectious disease transmitted from non-human animals to humans. Zoonotic spillover is the initial crossing of this species barrier. It is the primary cause of emerging infectious diseases — approximately 60% of known human infectious diseases are zoonotic, and the majority of recent pandemics have had animal origins (HIV from primates; influenza from birds/swine; Ebola from bats; SARS-CoV-2 from bats via possible intermediate hosts).

Key drivers of spillover: habitat destruction (increased human-wildlife contact), wildlife trade, intensive livestock farming, urbanisation, and global travel.
The conventional model assumed a "stepping stone" process: virus adapts in an intermediate host (e.g., civets for SARS, camels for MERS) before infecting humans at scale.
This new study's finding — that most viruses need no such stepping stone — means the window between first spillover and pandemic potential may be far shorter than previously assumed.
SARS-CoV-2: the study found no evidence of intensification or relaxation of selection across 15 genomic regions compared with the bat reservoir — consistent with direct or near-direct bat-to-human transmission.

Connection to this news: Dismantling the "intermediate host adaptation" assumption reshapes how surveillance systems should be designed — earlier detection at the animal-human interface becomes far more critical than monitoring viral evolution in intermediate animals.

One Health Framework: Integrating Human, Animal, and Environmental Health

The One Health approach — formally adopted by the WHO, FAO, UNEP, and WOAH (formerly OIE) as a Quadripartite framework — recognises that human health, animal health, and ecosystem health are inseparably linked and must be monitored and managed together. One Health is the principal conceptual framework for pandemic preparedness in the 21st century.

WHO definition: "One Health is an integrated, unifying approach to balance and optimize the health of people, animals, and ecosystems."
Established formally via the "One Health High Level Expert Panel" (OHHLEP) of the WHO-FAO-UNEP-WOAH Quadripartite (2021).
Key pillars: (1) Joint surveillance at animal-human interface; (2) Shared early warning systems; (3) Integrated response to outbreaks; (4) Addressing upstream drivers (deforestation, wildlife trade, antibiotic use in livestock).
India and One Health: India launched its "One Health" initiative under the National Health Mission, and India's National Action Plan on Antimicrobial Resistance (NAP-AMR) adopts One Health principles.

Connection to this news: The study's finding that spillover can lead directly to pandemic viruses reinforces One Health's core argument — surveillance and prevention must happen at the source (animal reservoirs and wildlife-human interfaces), not only after human cases appear.

Pandemic Preparedness: Global and India-Specific Dimensions

The COVID-19 pandemic demonstrated catastrophic costs of reactive pandemic management. Global mechanisms for pandemic preparedness include the WHO's Pandemic Accord (negotiations ongoing as of 2026), the Coalition for Epidemic Preparedness Innovations (CEPI), the Pandemic Fund (World Bank, launched 2022), and the G20 Health Ministers' framework. India's preparedness infrastructure includes ICMR, NCDC (National Centre for Disease Control), and the Integrated Disease Surveillance Programme (IDSP).

IDSP (Integrated Disease Surveillance Programme): India's real-time disease surveillance network covering all districts; maintains S- (syndromic), P- (presumptive), and L- (laboratory)-form reporting.
NCDC: coordinates outbreak response; functions as India's national public health institute (analogous to US CDC).
Pandemic Accord (WHO): negotiations to create a legally binding international instrument for pandemic prevention, preparedness, and response — ongoing as of 2026.
CEPI: established post-Ebola to fund vaccine development for epidemic-prone pathogens; played key role in COVID-19 vaccine development (AstraZeneca, Moderna partnerships).
The 1977 H1N1 exception in this study is significant: it is the clearest documented case of a laboratory-origin pandemic virus — relevant to ongoing biosafety and biosecurity policy debates.

Connection to this news: If most spillovers don't require pre-adaptation, pandemic preparedness systems need stronger animal-human interface surveillance (wildlife sentinels, wet market monitoring, livestock disease tracking) and faster human case detection — not just better treatments after outbreaks begin.

Key Facts & Data

Study: Published in Cell, March 2026; led by Prof. Joel Wertheim, UC San Diego.
Viruses analysed: SARS-CoV-2, SARS-CoV, H1N1 influenza (2009), Ebola (West Africa 2014), Marburg (Angola), mpox.
Key finding: No evidence of intensified selection pressure in animal reservoirs prior to human spillover — adaptation happens after sustained human transmission, not before.
Exception: 1977 H1N1 showed laboratory-passage genetic signatures — consistent with accidental lab release.
Zoonotic diseases: ~60% of known human infectious diseases; source of most major pandemics.
One Health: adopted by WHO-FAO-UNEP-WOAH Quadripartite; operationalised through OHHLEP (One Health High Level Expert Panel, 2021).
India surveillance: IDSP covers all districts with S-P-L form reporting; NCDC coordinates outbreak response.
Pandemic Fund (World Bank, 2022): $1.4 billion+ committed for pandemic preparedness in low- and middle-income countries.
COVID-19 origin implication (per study authors): "no evidence SARS-CoV-2 was shaped by selection in a laboratory or prolonged evolution in an intermediate host."