Hundreds of people have been sickened by the swine flu virus, prompting authorities to urge the public to get vaccinated, it was confirmed on Monday.

So far about 450 cases of swine flu, or the H1N1, have been identified in this year's outbreak. Among them two have died, one in Durban and the other in Pretoria, according to the Institute for Communicable Disease (ICD).

"But that is really the tip of the iceberg. The number of Influenza cases we are managing to identify in our surveillance system is slightly higher than it was last year," ICD director Shabir Madhi said.

"The important thing about swine flu virus right now is that because it's been circulating in the country for the past 3 to 4 years, the majority of people are actually immunized because of natural exposure to the virus," said Madhi.

H1N1 flu is a respiratory disease of pigs caused by type A influenza that regularly causes outbreaks of influenza among pigs. H1N1 flu viruses do not normally infect humans; however, human-to- human spread of H1N1 flu virus is occurring, similar to the spread of regular seasonal flu viruses.

H1N1 swine flu swept around the world in a 2009/2010 pandemic. The World Health Organization said the death toll reached 18,500 during that period. In South Africa, there were more than 5,000 cases and 40 deaths.

Most people are believed to have developed some immunity to the virus. But children, the elderly and patients with underlying conditions remain at risk, experts say.