Credit NIAID
#19,100
A number of countries have procured quantities of a pre-pandemic H5N1 vaccine for human use (United States (4.8 million doses), the UK (5 million doses), Canada (500,000 doses) Japan (10 million doses), and the EU (initially 664,000 doses but increased last year to 27+ million doses).
With the exception of a special release of 20,000 doses of H5N1 vaccine to Finland following their 2023 fur-farm epizootic (see Finland: MOH Announcement On Avian Flu Vaccine Availability For People At High Risk), these vaccines remain locked away, as nations debate under what circumstances, and who would receive them.
Since it is assumed that it would require two doses - given 30 days apart - to convey (likely limited) immunity, only a small percentage of the population can be vaccinated in the opening months of a pandemic.
It is widely expected that it could take 6 months or longer before any substantial quantity of vaccine would be available to the general public (see Maggie Fox's SCI AM - A Bird Flu Vaccine Might Come Too Late to Save Us from H5N1).
We've looked at various proposals for deploying existing pre-pandemic vaccines (see CMAJ: Avian Influenza and Use of the H5N1 Vaccine to Prevent Zoonotic Infection in Canada), but no strong consensus has emerged.
However, two groups are frequently mentioned.
- People who handle live avian influenza A(H5N1) virus in laboratory settings
- People with ongoing contact with known infected birds or other known infected animals or their environments
This week, Nature has published a study which considers some options on how best to contain a spillover of avian flu from poultry to humans (presumably farmers).
It is, alas, behind a paywall, but we do have a press release from York University to fill in the gaps. First the link to the study/abstract.
Containment scenarios for post-spillover transmission chains of avian influenza H5N1 from poultry to humans
Inspired by the Canadian teen who contracted H5N1 in the fall of 2024 without an obvious exposure risk (see NEJM: Critical Illness in an Adolescent with Influenza A(H5N1) Virus Infection) the authors modeled several approaches to containing a spillover to humans at a B.C. poultry farm.
Isolation of infected individuals is the first obvious step, but not every case will be symptomatic (see JAMA Open: Asymptomatic Human Infections With Avian Influenza A(H5N1) Virus Confirmed by Molecular and Serologic Testing).
Application of a `Tamiflu Blanket' may prevent some infections, but data on PEP (Post-Exposure Prophylaxis) is limited (see J.I.D.: Antivirals for Novel Influenza A Virus Infections), and antiviral resistant strains have been reported (see Emerg. Microbes & Inf: Oseltamivir Resistant H5N1 (Genotype D1.1) found On 8 Canadian Poultry Farms).
Reactive vaccination - performed after the outbreak has occurred - is hampered by the 30-40 days it takes to build immunity, whereas pre-emptive vaccination of high risk individuals (i.e. famers) negates that concern.
The authors write:
. . . reactive vaccination has very limited additional benefits outside of self-isolation, but pre-emptive vaccination adds substantial additional benefits on top of self-isolation.
I've reproduced some excerpts from the press release below. Follow the link to read it in its entirety. I'll have a bit more after the break.
Researchers model how to contain Avian flu H5N1 in case of human-to-human transmission
By Sandra McLean
March 27, 2026
At this point, Avian flu H5N1 is thought to have very limited ability to transmit between humans, but a recent case in British Columbia with an unknown source of transmission has piqued the curiosity and concern of scientists, including York University Professor Seyed Moghadas.
Did this lone case come about through transmission from an animal or another person, and if it was via human transmission, what methods will control its spread in the human population? Director of York’s Agent-Based Modelling Laboratory in the Centre of Excellence in AI for Public Health Advancement, Moghadas and a group of researchers used modelling to understand the best spread control measures should human-to-human transmission become possible.
“The idea was, let's evaluate some of the interventions that we usually implement at the very earliest stage of a disease outbreak or emerging disease, which we know very little about,” he says.
For the research, "Containment Scenarios for Post-Spillover Transmission Chains of Avian Influenza H5N1 from Poultry to Humans,” published today in Nature Health, various scenarios from isolation to vaccination before or after a spillover event were modelled. It is one of only a few studies that have explicitly modelled outbreak dynamics following spillover into humans or the effectiveness of public health interventions in early and highly uncertain phases of virus development.
(SNIP)
The researchers used Abbottsford, B.C. as the location as it is a highly dense poultry farming area. The starting point is after a spillover has happened.“If a human became infected, how do we block this single individual to trigger a large outbreak? Or if the infection is going on between humans, can we block these chains and to what degree we can block them?” asks Moghadas. “What is the effectiveness of either self-isolation of symptomatic cases or vaccination of farmers or vaccination of farmers and their household members?”
Even with mitigation measures, someone in the farmer’s family could potentially be infected by the farmer and then transmit it to someone in the community.
The team evaluated two different types of vaccination strategies. One was reactive, which means that you trigger a vaccination program after a case has been identified somewhere. The second strategy was pre-emptive – individuals, such as farmers, are vaccinated before any case is identified.
What they found is that reactive vaccination has very limited additional benefits outside of self-isolation, but pre-emptive vaccination adds substantial additional benefits on top of self-isolation.
Should the virus be confirmed to be capable of human-to-human transmission, Moghadas says they want to limit the chain of transmission and minimize the risk of evolution of the virus to become more adapted to human conditions. For now, he says, when cases are identified, the person should self isolate immediately. For the authorized vaccine, it should be meted out quickly to target populations, but that could take several weeks to have population level effectiveness.
“Timely action is a critical part of controlling the spread. Self-isolation of symptomatic cases has a significant effect, but that comes with the caveat that we don't know if everybody who is infected will develop symptoms,” says Moghadas.
“There could be potential asymptomatic cases we don't identify and by the time we do identify them, they've been already infecting others in the chain of transmission. This case in B.C. was particularly concerning because they could not find the source of infection.”
(Continue . . . )
There are obviously some formidable barriers to a pre-emptive vaccination campaign of farmers (poultry, and in some places, dairy) and their household contacts.
- Existing vaccine supplies are quite limited, and an argument could be made that vaccine supplies should be reserved for `essential workers' (e.g. lab workers, HCWs, military, etc.) after an outbreak occurs.
- Getting farmers to take the seasonal flu vaccine to prevent `reverse zoonosis' or worse - a co-infection with seasonal and zoonotic influenza - has been a challenge (see Canada: BC Provincial Health Officer Urges People Living/Working On Poultry Farms To Get Seasonal Flu Vaccine).
- Only 1 in 3 Canadians age 18-64 got the flu shot last year. It seems likely that an H5N1 vaccine would be an even harder `sell', particularly before any outbreak has happened.
- Farmers and governments may shy away from any program that hints that the food supply may be contaminated or unsafe, and `paralysis by analysis' may further delay any decisions.
