Although vaccinated children are known to shed the virus a few days after vaccination, the vaccine virus that is shed is less able to spread from person to person than the natural infection. The amount of virus shed is normally below the levels needed to pass on infection (transmit) to others and the virus does not survive for long outside of the body. This contrasts with a natural flu infection, which spreads easily during the flu season. In schools using vaccines, therefore, the overall risk of influenza transmission is massively reduced by having a large number of children vaccinated. In the US, where there has been extensive use of the nasal spray flu vaccine (live attenuated influenza vaccine; LAIV) for many years, serious illness amongst immunocompromised contacts who are inadvertently exposed to the vaccine virus has never been observed. Expert doctors at Great Ormond Street Hospital, who deal with many children with very serious immune problems, do not recommend keeping such children off school purely because of vaccination.