The following outbreaks, which occurred last year, illustrate the reality of zoonotic influenza, the fact that all ages can be vulnerable, that those with and without comorbidities can be at risk, and that various exposures can lead to swine influenza infection. It is interesting to note the way scientists are able to analyse the viruses and fit them within a phylogenic tree showing their relationships and evolution. The diversity of zoonotic influenza viruses that caused human infections in 2023 is alarming and infections of some types of zoonotic influenza viruses caused severe disease with a high mortality rate. In 2023 they did not transmit easily from person to person, although we never know when this may change, and therefore must be ever-ready for a pandemic.

WHO will continue to strengthen surveillance, jointly with its partners, in both animal and human populations, thoroughly investigate every zoonotic infection, build up pandemic preparedness planning, and get better readiness for the next influenza pandemic. Here is a summary of some swine influenza outbreaks notified to WHO last year:

Brazil

On 7 June 2023, WHO was notified of a fatal laboratory-confirmed human case of infection with a swine-origin influenza A(H1N1) variant (v) virus in the inner state of Paraná. The patient was a 42-year-old woman with underlying medical conditions who lived near a swine farm. She developed fever, headache, sore throat, and abdominal pain on 1 May 2023 and was hospitalized on 3 May with a severe acute respiratory infection. On 4 May, the patient was admitted to the Intensive Care Unit (ICU) and she passed away on 5 May. Ongoing investigations reported that the patient did not have any direct contact with pigs, however, two of her close contacts worked at a swine farm. The two contacts did not develop respiratory disease and tested negative for influenza. Sporadic human cases of influenza A(H1N1)v have been reported previously, including from Brazil.

Find out more about this outbreak here.

The United States of America

On 4 August 2023, WHO was notified of a human infection with a novel influenza A(H1N2) variant virus identified in the State of Michigan. The case was under 18 years old, with no comorbidities and was not hospitalized. Investigation by local public health officials identified swine exposure by the patient at an agricultural fair that took place between 23 and 29 July, within 10 days prior to illness onset. This was the first influenza A(H1N2)v virus infection identified in the United States in 2023. Since 2005, there have been 512 influenza A variant virus infections (all subtypes), including 37 (human infections with influenza A (H1N2)v viruses reported in the United States.

Find out more about this outbreak here.

The Netherlands

On 2 September 2023, WHO was notified of a laboratory-confirmed human case of infection with a swine-origin influenza A(H1N1) variant (v) virus in the province of North Brabant, Netherlands. The case was an adult with no underlying medical conditions and no history of occupational exposure to animals. As of 13 September, the person had recovered. This was the first human infection caused by influenza A(H1N1)v virus reported in the Netherlands in 2023. This case was picked up as part of routine surveillance of respiratory illnesses. Based on the available information, there was no clear indication of the source of infection, and no direct contact with pigs was reported.

Find out more about this outbreak here.

United Kingdom of Great Britain and Northern Ireland

On 25 November 2023, WHO was notified of a human case of swine-origin influenza A(H1N2) virus infection. The case reported onset of mild symptoms on 5 November. He visited his general practitioner on 9 November and as of 1 December was fully recovered. This case was identified after a respiratory sample was collected and further analyzed as part of routine surveillance of respiratory illnesses. According to ongoing investigations, pig farms were located within a few miles of where the case resided; however, the case reported no direct exposure to pigs, pets, or farms. The source of infection was under investigation and contact tracing was continuing. This influenza A(H1N2)v virus belonged to the swine influenza virus genetic clade 1B.1.1. Similar A(H1N2) viruses from this genetic clade have been previously detected in pigs in the United Kingdom. However, this is the first time a virus from this swine genetic clade has been detected in a human in the United Kingdom.

Find out more about this outbreak here.