LONDON, England (CNN) -- British airlines have put into effect measures to stop people with swine flu boarding flights in a bid to prevent the virus from spreading further.



Medical screening for the swine flu virus has been introduced at many airports.



British Airways said there had been a "very small number of cases" where people who had checked in with symptoms of H1N1 had been advised not to travel after having medical checks.



Virgin Atlantic also said victims would not be allowed to board one of its planes without a fit-to-fly certificate from their doctor or a hospital, though there had been no cases yet.



The World Health Organization declared the virus a global pandemic June 11. More than 120 countries have reported cases of human infection. About 98,000 cases have been documented worldwide, with 440 deaths, according to the WHO.  Watch as airlines ban sufferers »



With 29 deaths and a huge rise in the number of cases, Britain has the worst swine flu figures in Europe.



Eight British schoolchildren remained in hospital in China on Monday after contracting swine flu on a trip to the country, the Foreign Office said. The teenagers were diagnosed with the H1N1 virus in Beijing. More than 50 of their classmates and teachers are also quarantined in a hotel.  Watch as students are quarantined »



Medical screening for the swine flu virus has been introduced at many airports around the world for passengers arriving on international flights but there are concerns that many people may not be aware they are infected.



Those who do have symptoms have been advised by Britain's health authorities to delay their journeys until the signs have cleared up.



"We have a medical team within the airline as well as a contingency planning group which has met for the past few years to look at the issue of a flu pandemic," A British Airways spokeswoman said.



"We have a wide range of contingency plans in place which we can use depending on how the situation may evolve.



"If we have concerns about a customer or the customer is concerned, then we have a 24-hour medical service we can call to give advice to staff.



"They will speak to the customer and an assessment will be made about their fitness to fly.



"There have been a number of cases where we have advised customers not to fly on the basis of their diagnosis or symptoms of H1N1."  Watch as a CNN reporter is quarantined »



BA told CNN Monday though that it was "business as usual" and all flights were operating normally.



Virgin Atlantic spokesman Paul Charles said: "If there are signs of something being wrong, be it excessive sneezing or coughing, not looking well, high temperature, then the airport staff can call in a medical team for extra advice.



"If the medical team believe there are reasons not to fly, the passenger will be asked to produce a fit to fly certificate from their doctor or a hospital, and they will be put at our cost on to the next available flight."



Swine flu has spread so rapidly and extensively around the globe that the World Health Organization is changing tactics against the H1N1 virus, including stopping a tally of cases and focusing on unusual patterns.



"At this point, further spread of the pandemic, within affected countries and to new countries, is considered inevitable," the WHO said.



The counting of all cases is no longer essential because it is exhausting countries' resources, the organization said.



"In some countries, this strategy is absorbing most national laboratory and response capacity, leaving little capacity for the monitoring and investigation of severe cases, and other exceptional events."



Monitoring is still required, the organization urged, but should focus on exceptional patterns.



"Because the numbers of cases have increased in so many countries, it is very hard to keep up," Keiji Fukuda, WHO assistant director-general, said earlier this month.



Laboratories have been inundated with testing requests and the virus is showing up in most lab tests in countries with major outbreaks, he said.



The organization said it will
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