Research and analysis

Technical risk assessment as of 30 January 2023 for avian influenza (human health): influenza A H5N1 2.3.4.4b

Updated 25 July 2024

Applies to England

UK virus population

There is an increase in confirmed cases of influenza A in birds (high confidence). In 2022, there was year-round maintenance of influenza infection in indigenous wild birds which represents a change compared to the usual seasonal pattern in which infections die out over the summer. Compared to the previous risk assessment of 13 December 2022, there are a reducing number of infected premises following the introduction of the national housing order for farmed poultry, but still high levels of detections in dead wild birds.

Influenza A H5N1 is the predominant influenza virus subtype detected in wild birds and farmed flocks in the UK (high confidence). There is diversity within the UK population of H5N1 viruses with 12 genotypes detected since October 2021, including some reassortment with low pathogenic avian influenza viruses (LPAIVs). The dominant circulating genotypes since October 2021 have been AIV09 and AIV07-B2. Since October 2022, AIV09 is increasingly predominant although the sample of wild birds is unlikely to be population representative. Another currently detected genotype in poultry is AIV48 which includes genes from gull-associated influenza viruses.

Genomic surveillance is proportionate for poultry outbreaks (a genome is generated for every affected premise). There is a limited genomic surveillance sampling in wild birds. The Animal and Plant Health Agency (APHA) select birds to test and report that testing is distributed in time and space with host species consideration. Expanded mammalian surveillance has commenced. Genomic data lags 7 to 10 days behind date of sample collection for poultry and currently longer for mammals.

Extent of human exposure in the UK

Owing to the disease burden in birds, there is an increased interface between humans and infected birds (high confidence). The high number of wild birds and domestic flocks with influenza A infection, especially where personal protective equipment is not worn, increases the likelihood of human exposures to this virus (moderate confidence).

Propensity to cause mammalian and human infection

Available surveillance data reported by APHA do not suggest widespread mammalian adaptation of this virus (low to moderate confidence).

Mutations known to be advantageous in mammalian infections are infrequent in the available genomic data from avian viruses. APHA report that there is evidence of direct spill over from birds into some ‘scavenger’ wild mammalian species within the UK (and others noted outside the UK). The species affected (foxes and otters) are presumed to have direct high-level exposure to infected birds based on feeding behaviour and food preferences.

In 2022, 56 mammals were tested, of which 8 were positive. The 4 available influenza genomes from these positive mammals all show the PB2 E627K substitution. This mutation is known to be acquired rapidly after infection of a mammalian host in some influenza viruses and is associated with enhanced polymerase activity. The rapid and consistent acquisition of the PB2 mutation in mammals may imply this virus has a propensity to cause zoonotic infections and further assessment should be made of the properties of this mutation.

Enhanced mammalian surveillance was initiated in January 2023, targeting mammals found dead near known areas of avian influenza transmission. In 2023 to date 2 animals have been tested, of which one fox was positive.

There are confirmed mammalian infections reported from multiple other countries, including multiple species in the USA, a cat in France, and a large outbreak in farmed mink in Spain. The available genomes from the mink include a different PB2 mutation to that seen in UK mammals.

There have been very small numbers of influenza A H5N1 2.3.4.4b detections in humans given the likely number of exposures. There is limited asymptomatic testing of human contacts of bird cases in the UK and international surveillance is variable. Nevertheless, by comparison with other zoonotic infections including influenza viruses, these data suggest that zoonotic infections are infrequent (low confidence).

Ability to cause (a) severe infection and (b) asymptomatic infection in humans

There is insufficient information to judge the risk of either severe or asymptomatic disease.

Human-to-human transmission

There is no evidence of sustained human to human transmission (moderate to high confidence). Surveillance based on clinical testing data and systems designed for seasonal influenza monitoring are unlikely to detect rare emerging influenza events. There is insufficient information to assess the occurrence of limited human to human transmission such as transmission within households.

The current H5N1 2.3.4.4b viruses in UK birds react well against antisera raised against an available Influenza A(H5) candidate vaccine virus (CVV) (A/Astrakhan/3212/2020), developed for pandemic preparedness under coordination from the World Health Organization (WHO).

Assessment

The avian influenza outbreak in the UK is assessed as at risk level 3*. The apparent transmission between mink is of significant concern but there is no clear evidence that this has continued in mammalian species since the initial outbreak. The UK assessment is consistent with the international picture with the same or similar viruses detected in multiple regions, evidence of mammalian spillover, but very few detected human infections. At present, there are no indicators of increasing risk to human health, however this is a low confidence assessment. The risk assessment is dynamic and requires regular review during this period of unusually high levels of transmission in birds with mammalian spillover.

*Levels are described in Investigation into the risk to human health of avian influenza (influenza A H5N1) in England: technical briefing 1.