Chickenpox vaccination is now fairly widespread across the USA, with claims of 95 to 100 per cent effectiveness being made for it in the medical press.
So doctors in New Hampshire were recently surprised to discover that over 30 per cent of children in a daycare centre developed chickenpox, despite many of them having previously been vaccinated.
Indeed, the very child who had caused the outbreak had been vaccinated three years earlier (N Engl J Med, 2002; 347: 1909-15).