Four more people in East China have been confirmed to be infected with the H7N9 bird flu virus, as a nationwide screening program has been launched for the rare but lethal strain.

The health authority in Jiangsu province said four people living in different cities are in critical condition after they developed fever, coughing and other symptoms around March 20. They tested positive for H7N9 on Tuesday afternoon.

Two men from Shanghai, aged 27 and 87, were confirmed dead on Sunday from H7N9 infection, with a woman from Anhui province in critical condition.

No epidemiological links have been found among the four cases announced on Tuesday, the Jiangsu health authority said.

Hundreds of people who have had close contact with the patients have not, as yet, developed fever or respiratory symptoms.

Feng Zijian, director of emergency response at the Chinese Center for Disease Prevention and Control, said the country has launched a nationwide program to screen for H7N9 among patients with pneumonia, where the cause is unknown.

The program, supported by surveillance systems at public health institutions including hospitals, is aimed at timely detection of new cases.

To determine the source of the virus, the identities, background and living conditions of existing patients will be investigated, Feng said. “It’s unclear if the virus originated in pigs or other animals,” he added.

The Jiangsu patients are three women aged 32, 45 and 48 and an 83-year-old man. They live in Nanjing, Suqian, Suzhou and Changzhou.

Only the 45-year-old woman, who works as a poultry butcher in Nanjing, has been in close contact with poultry, according to the health authority report. The others work in sheet metal processing or are unemployed.

The health department has set up a team to direct prevention and control of the virus.

Sixteen top-level hospitals in Jiangsu have been designated as treatment sites for patients confirmed to have the virus, and all levels of hospitals are required to strengthen the screening of pneumonia patients against H7N9.

Other cities, including Shanghai and Beijing, have put forward plans to deal with emergencies or large-scale outbreaks caused by the relatively unknown strain of bird flu.

Shanghai authorities on Tuesday launched a yellow warning third-level emergency in response to the new strain.

The yellow warning suggests that people are infected with the virus but that human-to-human transmission has yet to take place.

Xu Jianguang, director of the city’s health and family planning commission, said at a news briefing on Tuesday that hospitals across Shanghai will strengthen the role of fever diagnosis wards to isolate patients.

Hospitals must also take preemptive measures, such as proper disinfection and quarantine, and step up protection of medical workers.

Experts say the manufacture of a vaccine will take some time, as researchers will need a thorough understanding of the virus.

No new pneumonia cases from unidentified sources have been reported in Shanghai, said Wu Fan, chief of the city’s Municipal Center for Disease Control and Prevention.

Officials said poultry and pork for sale at local markets are safe, but people are urged not to eat wild poultry.

On Tuesday, Shanghai authorities reiterated that no bird flu virus had been found in dead pig samples from a river providing drinking water to residents.

The Shanghai Animal Disease Prevention and Control Center tested 34 samples from pig carcasses pulled from the Huangpu River and found no bird flu viruses.

Thousands of dead pigs were fished from the river last month, sparking fears over tap water safety.

Officials denied they had covered up the two bird flu deaths, which occurred on March 4 and 10.

Wu said it is difficult and takes time to detect an unknown virus. It is still not known if the virus can be transmitted between poultry and other animals or between pigs and humans, she added.

In Beijing and Guangdong province, public health authorities said they are keeping a close eye on the H7N9 virus.

Test reagents for the virus have reached Beijing — where the deadly SARS epidemic struck a decade ago — according to the city’s health bureau.

The capital, with a population of about 20 million, has added bird flu to its monitoring system for regular flu and pneumonia, where the source is unclear.

The authority said all hospitals in Beijing have been told to brace for emergencies and ensure enough medical supplies, although no infections had been reported in the city as of Tuesday.

It is the first time the H7N9 bird flu virus has been found in humans, although three other strains in the H7 family — H7N2, H7N3 and H7N7 — have previously been found in humans, but there were no casualties.