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In a follow-up on the recent Lassa fever cases reported in England (HERE and HERE), the World Health Organization (WHO) provided more details in a statement released Monday.

This transmission electron micrograph (TEM) depicted numbers of Lassa virus virions adjacent to some cell debris. Image/C. S. Goldsmith

On 9 February 2022, WHO was notified by the United Kingdom health authorities of two laboratory confirmed cases and one probable case of Lassa fever. As of 18 February, three cases have been confirmed and one death has been reported. These are the first reported Lassa fever cases in the United Kingdom since 2009, and represents the second known case of secondary transmission of Lassa fever in Europe.

The first case travelled to Mali in late 2021, where Lassa fever is endemic. After returning to the United Kingdom, the individual developed symptoms of fever, fatigue and loose stool, and was subsequently hospitalized. The first case has now recovered. The second and third cases were family members of the first case and did not travel to Mali. Both were admitted to the hospital and the third case has sadly died.

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On 8 February, samples from both the first and second cases were laboratory confirmed positive for Lassa virus by PCR. The third case was initially provided care as a probable case of Lassa fever and the diagnosis was confirmed on 9 February.

Incident responses have been activated by health authorities in the United Kingdom to coordinate the clinical and public health management of these cases.

People with exposure to the cases were identified through robust contact tracing activities. Low and high risk contacts are being followed up for 21 days after their last exposure. Post-exposure prophylaxis has been offered to high-risk contacts with the most significant exposures. Systems are in place to assess and manage any contacts who may develop symptoms during the follow up period.

Lassa fever is endemic in Mali and sporadic cases have been exported to Europe from countries in West Africa where the disease is endemic, such as Sierra Leone, Togo, Liberia and Nigeria in recent years. However, the secondary transmission of Lassa fever through human contacts is known to be extremely rare when appropriate infection prevention and control precautions and laboratory biosafety measures are applied. The only secondary transmission of Lassa fever reported in Europe was in 2016 from Germany, when the secondary case performed post-mortem care of a fatal case.

Human-to-human transmission occurs in both community and health care settings, where the virus may be spread by contaminated medical equipment, such as reused needles, as well as direct contact with bodily secretions. Health care workers are at risk if caring for Lassa fever patients in the absence of appropriate infection prevention and control (IPC) practices, although information from recent imported cases in Europe show that secondary transmission of Lassa virus is rare when standard IPC precautions are followed. The possibility of additional secondary cases related to transmission in a health care setting cannot currently be excluded, but is considered very unlikely in the community.