Infectious Diseases 2022

UK virus population: There is an increase in confirmed cases of influenza A infected birds. In 2022, there has been year-round maintenance in indigenous wild birds, which represents a change compared to the usual seasonal pattern in which infections die out over the summer.
Influenza A H5N1 is the predominant influenza virus subtype detected in wild birds and farmed flocks in the UK. There is diversity within the UK population of H5N1 viruses with 10 genotypes detected since October 2021, including some reassortment with low pathogenic avian influenza viruses (LPAIVs). The dominant circulating genotypes are currently AIV09 and AIV07-B2. The other currently detected genotype in poultry is AIV48, which includes genes from gull-associated influenza viruses.
Extent of human exposure in the UK: Owing to the disease burden in birds, there is an increased interface between humans and infected birds. The high number of wild birds and domestic flocks with influenza A infection also increases the likelihood of human exposures to this virus without personal protective equipment.
Propensity to cause mammalian and human infection: Available surveillance data does not suggest widespread mammalian adaptation of this virus. Known mammalian adaptation mutations are infrequent in the available genomic data from avian viruses. There is evidence of direct spillover from birds into some mammalian species. The species affected (foxes, otters and seals) are presumed to have direct high-level exposure to infected birds based on feeding behaviour and food preferences. Few mammalian infections have been detected. The 8 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. There is an unconfirmed recent report of possible transmission between mink in Spain. 
There have been 4 instances of influenza A H5N1 2.3.4.4b detection in humans (one in the UK, one in the US, and 2 in Spain). 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, this data suggests that zoonotic infections are not frequent.
Ability to cause (a) severe infection and (b) asymptomatic infection in humans: There are no detected severe human cases associated with influenza A H5N1 (clade 2.3.4.4b) in the UK or internationally. There is insufficient information to judge the risk of asymptomatic or mild disease due to limited testing in human contacts of infected birds.
Human-to-human transmission: There is no evidence of sustained human-to-human transmission. 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 World Health Organization candidate vaccine virus (A/Astrakhan/3212/2020).
Assessment: The avian epidemic has passed the triggers for undertaking enhanced characterisation and surveillance. 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. In vitro and animal model data is required, and detailed sentinel human infection surveillance is recommended.