EDMONTON (CBC) - Four more people in Alberta have died from complications of H1N1, health officials announced Thursday, bringing the total deaths in the province to 18 since the outbreak began in April.

The deaths occurred in Edmonton, Calgary and the southern and central regions of Alberta. Three of the individuals were known to have risk factors. Two were middle-aged and the other two were elderly, said Dr. Gerry Predy, Alberta Health Services' senior medical officer of health.

"Given the amount of influenza that's circulating the community now, having four deaths this week is not unexpected," Predy told reporters at a news conference in Edmonton on Thursday morning. "It is tragic. It's unfortunate when anybody dies."

As has been the current practice in the province, no additional details were provided on the people who died.

The number of hospitalizations due to the flu has also shot up: 439 cases as of Thursday, compared with the 314 people who were reported in hospital as of Tuesday.

About 25 per cent of the people in the province's intensive care units are either confirmed to have or suspected of having the virus, Predy said.

The announcement comes as Alberta resumed its H1N1 vaccination program Thursday morning after it was abruptly shut down on the weekend due to high demand and a national vaccine shortage.

On Thursday, vaccinations will only be offered to children under five years old and older than six months of age. The program will be expanded to include pregnant women on Friday.

An additional 275,000 doses of the vaccine are available until the end of next week, which is believed to be enough to vaccinate children under five and pregnant women.

Health officials won't say which high-risk group will be given priority to get vaccinated next.

Since the province decided to restrict who gets the vaccine, other groups like police officers have complained that they haven't been able to get access to the vaccination.

During Thursday's news conference, Predy acknowledged people's concerns.

"We do have sympathy. These are not easy decisions with respect to [prioritizing] the high priority groups," he said. "We know there are others at high risk of getting influenza but what we're focusing on are those at high risk of getting more severe illness."