Four more people in southern China have been reported as being infected with the little-known H7N9 bird flu, as health authorities on Tuesday placed hospitals around the country on alert amid new concerns about a rare virus that has never before been reported in humans.

At least seven people have, so far, been reported as contracting H7N9. Two deaths have already been reported in Shanghai, while a third person is in a critical condition in nearby Anhui province. There is no known vaccine to the H7N9, which is a rare form of avian flu.

The four new cases were reported on Tuesday in Jiangsu province, which borders Anhui to the east and lies north of Shanghai. The Jiangsu health bureau said three women, aged 45, 48 and 32, and a man (83) had been infected. Worryingly, the four cases were in four different cities, and all patients were in “critical condition” and “under emergency treatment”, Xinhua reported.

The new cases have put health authorities here on alarm, with hospitals told to prepare for emergencies, Xinhua said, to avoid a repeat of the SARS outbreak 10 years ago. All hospitals in Beijing “have been asked to brace for emergencies” and the avian flu had been added to an existing monitoring system, Xinhua said.

The fresh concerns about the H7N9 come in the wake of another public safety scare in recent weeks in Shanghai, where the carcasses of thousands of pigs were found floating on the Huangpu river. That the deaths of the pigs were reported to have taken place upstream, in the nearby provinces of Zhejiang or Jiangsu, has led to speculation that the cases may have been linked to the spread of H7N9.

Authorities, however, said on Tuesday no bird flu virus had been found in 34 samples of the pigs tested so far. Looking to calm fears, health officials said there was no known case of H7N9 being spread through human-to-human contact. While reports said several relatives of one of the men who died in Shanghai had also suffered from pneumonia in recent days, local officials said those cases were unrelated.

An 87-year-old man and a 27-year-old died on March 4 and 10 respectively after contracting the virus in February. The cases were confirmed as H7N9 only on Saturday. The delay, officials said, was because the cases had to go through a series of laboratory tests and epidemiological surveys as this rare form of avian flu had never before been found in humans.

That two people had died and five others were said to be in a critical condition has indicated the virulence of the flu, though authorities said it was impossible to state with certainty either the strength or transmission capabilities of H7N9. There is also no vaccine for the little-known flu.

“It has shown, for the time being, no human-to-human transmission,” said Fadela Chaib of the World Health Organisation (WHO). “We need to further investigate the extent of the outbreak, the source of the infection and the mode of transmission”.