[monocle]

At the very least all employees of mass transportation, particularly air transport, should be required to be vaccinated. If a terrorist were infected and then travelled to the US via an airline, the ticket processors, airline stewards, custom agents and immigration agents could become carriers and infect all travellers they came in contact with, these travellers in turn would disperse throughout others in many parts of the country.

[UKCajun]

Heathrow and Gatwick are gateways to the the U.S.!