CHICAGO, May 18 — An Illinois resident tested positive for Middle East Respiratory Syndrome after being in contact with an infected patient, though he did not show signs of illness, US health officials said yesterday.

The man likely contracted MERS from a man in Indiana who was hospitalised in late April with the first known US case of the illness.

The Illinois resident’s lack of symptoms may shed light on milder forms of the deadly virus, which emerged in the Middle East in 2012 and has infected more than 500 patients in Saudi Arabia alone. It kills about 30 percent of those who are infected.

Researchers at the forefront of the global MERS response said this week they were investigating whether people infected with MERS who have no symptoms could still pass the virus on to others.

“There is evidence there is a broader spectrum of MERS” than first expected, said Dr David Swerdlow of the US Centers for Disease Control and Prevention, who is leading the US response to MERS.

The Illinois resident did not seek or require medical care and is reported to be feeling well, but officials involved in investigating the first case have been monitoring his health since May 3. A blood test on Friday showed he had developed antibodies to MERS.

CDC officials explained that the blood test is not sufficient to consider him a confirmed case of MERS because it detected only antibodies, not the live virus. Swerdlow said the agency would discuss with the World Health Organisation its system of classifying MERS cases to account for milder cases.

On April 25, the Illinois man had a 40-minute face-to-face meeting with the Indiana patient, a business associate, Swerdlow said. The two men shook hands but the Indiana patient did not have a cough at the time.

The Illinois resident has been instructed to avoid other people or wear a face mask. While the Indiana patient was a healthcare worker who had recently arrived in the United States from Saudi Arabia, the Illinois resident had no recent history of travel outside the country, Swerdlow said. — Reuters