However, Health Secretary Francisco Duque III said there is no cause for alarm as the five workers had antibodies that indicated they have developed an immunity to the virus.

"Hindi kailangan mangamba basta gawin ang apat na tagubilin ng DOH (There is no cause for worry so long as you follow the four steps the health department has prescribed)," Mr Duque said in an interview on dzBB radio.

GMA News.tv reports that Mr Duque said the steps include keeping infected hogs out of the food chain, taking bio-safety and bio-security measures, making sure pork is cooked, and prevention of double-dead meat.

Consumers can call a hotline for the Ebola Reston virus at (02)925-99-99, he said.

Radio dzBB's Carlo Mateo quoted National Epidemiology Center head Eric Tayag as saying three of the five were from affected hog farms, one from the Valenzuela area, and one from a backyard farm.

Mr Tayag said they are still investigating how the five were exposed to the virus.

Mr Duque said the four found positive for exposure to the Ebola Reston virus had normal immune systems and do not need to be quarantined.

"Kaya nilang tibagin ang virus, hindi na sila kailangang i-quarantine.

Hindi sila infectious (They have an immunity against the virus, they do not need to be quarantined. They are not infectious)," he said.

The four Filipinos were among the 77 who gave blood samples to health workers.