The worrying developments emerged as a 37-year-old man critically ill in hospital in Glasgow was yesterday confirmed as having swine flu, and five members of his family are being viewed as possible cases.
The man, who has not been named, was admitted to intensive care last week and has a serious underlying illness. He is not known to have had contact with any swine flu cases or travelled recently.
The total number of confirmed cases in the UK leapt on Tuesday after an outbreak at Welford Primary School in Handsworth, Birmingham infected 50 children. One further case was reported in the East Midlands yesterday taking the UK total to 185 cases.
In the US, where the official number of cases stands at 6,764, including 10 deaths, experts at the Centres for Disease Control and Prevention say the true total is likely to be 20 times higher, at over 100,000. The official total is the "tip of the iceberg", they said.
National surveillance systems in the US and Europe depend on family doctors taking swabs from patients who consult them with flu-like symptoms and sending them to the laboratory for analysis, but most patients with flu never see a doctor.
A spokesman for the UK Health Protection Agency (HPA) said the size of the outbreak at Welford Primary School was "not unexpected" and there was "no evidence of sustained transmission in the community at the moment".
He added: "It is the largest school outbreak so far. But if we get a case in a school we know it will spread. Seasonal flu is no different. We fully expect more cases in the same school."
Officials accept that if the novel H1N1 virus starts to spread rapidly, the government will be forced to abandon its containment strategy, which involves offering anti-viral drugs prophylactically to close contacts of those infected.
The HPA spokesman said: "We are using [the containment strategy] at the moment. It buys you time to develop a vaccine by delaying the spread. I don't think anyone assumes you can go on with this indefinitely, but we have not reached that point yet."
A spokesman for the European Centre for Disease Prevention and Control (ECDC) in Stockholm, which monitors the spread of swine flu across Europe, said: "With any surveillance system there will be cases that are not picked up. The number of laboratory confirmed cases is likely to be lower than the total in the real world."
"The strategy of containment is being pursued by all countries in Europe but, at some point, that will change. You could paint a very rosy scenario of the whole thing burning out but that is unlikely."
Over 50 countries have confirmed cases of swine flu totalling 13,394 people, including 95 deaths. Although most of these have suffered mild flu, the virus is putting a number of children, young adults and pregnant women in hospital.
Among 30 patients hospitalised with swine flu in California – with an average age of 27 – half had signs of pneumonia, six were treated in intensive care and four required ventilation. Five were pregnant, two of whom developed complications and spontaneously aborted. Almost two-thirds (19) had an underlying medical condition that may have lowered their immunity.
Experts from the ECDC said although the evidence was preliminary, it showed older people may have some protection against swine flu, which could include elements from flu viruses circulating in the 1960s, to which they may have developed immunity. Younger people, especially those with chronic illnesses or pregnant, may by more susceptible and need priority treatment with anti-virals and vaccination, they said.
Will a more dangerous strain return in autumn? Are we really ready?
Why has swine flu not spread further?
Fewer than 200 people have been identified with the virus in the UK in more than a month; normal flu outbreaks cut a swathe through populations in days or weeks. However, more may have been infected but not felt ill enough to bother their GP – in the US health officials reckon the true total is 20 times the recorded number.
Is it about to take off in the UK?
Unlikely during the warmer summer months. Flu viruses spread more readily in damp, humid conditions and swine flu is expected to return – and spread more widely – in the autumn.
Is it as mild as it looks?
Yes – but this could be deceptive. We know it is a mixture of pig, bird and human viruses in a combination never seen before. It has jumped the species barrier and is spreading from human to human – that alone is disturbing. Even "mild" seasonal flu infects 10 per cent of the population and kills around 4,000 people in the UK per year.
Could it become more dangerous?
Yes – this is what keeps Government chief medical officers awake at nights. The virus is constantly evolving and could mutate into a more aggressive form. This is what happened in the 1918 pandemic and scientists are looking carefully to detect any changes.
Is a pandemic inevitable?
Most experts agree – it is only a matter of time. The critical unanswered question is how bad it will be – both in the number of cases and the severity. A "pandemic" does not automatically imply severe disease. If the virus evades the current strategy of containment, as is likely, and starts to spread, the Government will face a tough challenge explaining why it is withdrawing prophylactic treatment with anti-viral drugs for contacts of infected people.
Swine flu: Country by country
US – 6,764 cases
Mexico – 4,541 cases
Canada – 921 cases
Japan – 360 cases
UK – 185 cases
Spain – 138 cases
Chile – 86 cases
Panama – 76 cases
Australia – 39 cases
Costa Rica – 33 cases
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