HANOI, June 15 (Reuters) - Many sick passengers who flew to Ho Chi Minh City in southern Vietnam took fever reducers to cheat temperature scanners at the airport, leading to the discovery of several infected cases later, state media reported at the weekend.

Nguyen Van Chau, head of Ho Chi Minh City's Health Department, was quoted in state-run Tien Phong (Vanguard) daily as saying "a series of passengers" took fever reducers three hours before arrival.

"That's why when they passed through the airport, the body temperature scanners skipped them," Chau said.

Vietnam has confirmed 26 people infected with H1N1 flu, 23 of whom came by air after travelling in the United States or Australia. Three others fell sick after coming in contact with the infected travellers.

The country requires inbound passengers found with fever to be isolated for seven days.

Clinical investigations of the infected cases revealed they had taken fever reducers, Chau said.

The World Health Organisation (WHO) declared an influenza pandemic on Thursday and advised governments to prepare for a long-term battle against the virus.

The H1N1 strain has spread widely, with 28,774 infections confirmed in 74 countries to date, including 144 deaths, according to the WHO's latest tally. Vietnam has so far had no deaths from the flu.