H7N9 influenza A, a strain of avian flu not previously known to infect humans, has sickened seven and killed two in China, according to health authorities.

Two of the first three cases reported to WHO on March 31 have died and the other remains in critical condition. Among four additional cases identified in China on April 2, none have died, but all are in intensive care.

Chinese health officials are increasing surveillance, and investigations into the source of infection and mode of transmission are ongoing, according to WHO.

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Thus far, all cases have presented with respiratory tract infections with progression to severe pneumonia and breathing difficulties. No epidemiological link between the seven cases has been identified, and no further cases have been identified among 88 identified contacts of the first three cases, or among the 167 close contacts of the additional four cases.

H7 influenza infection in humans is uncommon but has been documented in people who have been in direct contact with infected birds, according to the Chinese CDC. However, the disease course in humans has been mild in the past, with illness consisting of upper respiratory disease and conjunctivitis.

These are the first reported cases of human infection with H7 influenza in China. H7N9, specifically, has only been previously isolated in birds, and outbreaks among birds have only been reported in Netherlands, Japan and the United States.

Two of the cases in the current outbreak occurred in Shanghai, one in Anhui province and the remaining four in Jiangsu province.

“The Chinese government is actively investigating this event and has instituted enhanced surveillance, laboratory strengthening and training of health care professionals for detection, reporting and treatment,” WHO said in a press release.