The United Nations’ senior health officer has warned that the spread of H1N1, so-called swine flu is now unstoppable. Dr Margaret Chan delivered her bleak assessment as Britain predicted 100,000 new flu cases a day in the UK by the end of August, and said it was time to change tactics. Globally, 332 people have died, mostly people with underlying health problems, but fit young adults are also dying.“With well over 100 countries reporting cases, once a fully-fit pandemic virus emerges its further international spread is unstoppable,” said Dr Margaret Chan, the WHO director general. The news came as flu claimed a fourth life in Britain. The victim was a teenager from London with serious existing illnesses. He is the first person to die in the capital since the virus spread from Mexico in April. “We have had larger numbers every day and for the last ten days or so we have been following a policy in London of doctors using their clinical judgement rather than swabbing and testing every patient,” said Simon Tanner, Director of Public Health in London. “That is now the position for the whole of the UK.” A pandemic vaccine should be available by the end of the summer, although sufferers continue to be treated with Tamiflu. Reports that a man in Denmark, a Japanese woman and now a teenage girl in Hong Kong have become resistant to the drug’s effects have been accepted by its makers Roche as being within clinical expectations.