Avian influenza has been confirmed in two poultry workers after they came into contact with infected birds, the UK Health Security Agency (UKHSA) has said.
The two people returning positive tests are known to have recently worked on an infected poultry farm in England, UKHSA said on Tuesday evening (16 May).
Neither has experienced any symptoms of avian influenza and both have since tested negative.
The agency said it detected the virus following the introduction of an asymptomatic testing programme for people who have been in contact with infected birds.
Detection of avian influenza in poultry workers can follow contamination of the nose and throat from breathing in material on the affected farm or can be true infection.
UKHSA said it can be difficult to distinguish these in people who have no symptoms.
Based on the timing of exposures and test results, it said one individual is likely to have had contamination of the nose and/or throat from material inhaled on the farm, while for the second individual it is more difficult to determine which is the case.
Further investigation is under way, but precautionary contact tracing has been undertaken for the second individual.
UKHSA said it has not detected evidence of human-to-human transmission and these detections do not change the level of risk to human health, which remains 'very low'.
Professor Susan Hopkins, chief medical advisor at UKHSA, said current evidence suggested that bird flu viruses do not spread easily to people.
"However, we know already that the virus can spread to people following close contact with infected birds," she said.
"This is why, through screening programmes like this one, we are monitoring people who have been exposed to learn more about this risk.
"Globally there is no evidence of spread of this strain from person to person, but we know that viruses evolve all the time and we remain vigilant for any evidence of changing risk to the population."
In the asymptomatic surveillance programme, workers are asked to take swabs of their nose and throat which are tested for the presence of influenza virus, during the 10 days after their exposure.
In some cases they may also be asked to have finger prick blood tests to see if UKHSA can detect antibodies against avian influenza, suggestive of an immune response in the blood.
As part of public health response, the agency follows up all individuals who have been in contact with a confirmed human case of avian influenza.
For those with the highest risk exposures, it may offer testing and antivirals, to help protect them from infection as well as to reduce the risk of passing infection to others.