The swine flu virus has reached 160 countries and could infect two billion people within the next two years, the World Health Organization has said. A senior WHO official, Keiji Fukuda, said the virus was still in its early stages and would continue to spread for some time. Mr Fukuda said work on a vaccine was intensifying but safety could not be compromised by rushing the process. The virus is thought to have killed almost 800 people in recent months. Mr Fukuda, the WHO's Assistant Director General for Health Security, said the agency had been reporting only laboratory-confirmed cases, but that this was always going to be "only a subset of the total number of cases". "Even if we have hundreds of thousands of cases or a few millions of cases, we're relatively early in the pandemic," he told the Associated Press news agency. The situation in Australia, the US and Germany "One of the things that is relatively clear is that we will continue to see spread of the virus; even though we are now three to four months into the pandemic, this is still pretty early into the overall period," he said. Mr Fukuda said the WHO estimates two billion people, one third of the global population, could eventually be infected. He said the figure was a reasonable prediction, based on analysis of previous pandemics, but that it was "really impossible to predict what the future will hold". Pregnancy risk World response to swine flu crisis Correspondents' round-up Send us your comments Mr Fukuda said officials and drug manufacturers were investigating how to speed up the process of developing a vaccine against the H1N1 swine flu strain. But he said there could be no doubt over the safety and efficacy of the drug before it was publicly distributed. "There is always a balance in this sort of situation. You of course want to get out vaccine and as much vaccine as possible, as quickly as possible. On the other hand there are certain things which cannot be compromised," he said. "There are certain areas where you can make economies, perhaps, but certain areas where you simply do not try to make any economies." The WHO says that in most affected countries, the majority of cases appear to be occurring in young people, around the ages of 12 to 17, although some reports suggest it is mainly older people who have required hospital treatment. The organisation also said there was "accumulating evidence suggesting pregnant women are at higher risk of more severe disease". But Mr Fukuda said the WHO "certainly has no recommendations on whether women should try to have children now".