Health Protection Agency, Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK. ali.al-shahib@hpa.org.uk|Health Protection Agency, Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK. ali.al-shahib@hpa.org.uk|Health Protection Agency, Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK. ali.al-shahib@hpa.org.uk|Health Protection Agency, Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK. ali.al-shahib@hpa.org.uk|Health Protection Agency, Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK. ali.al-shahib@hpa.org.uk|Health Protection Agency, Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK. ali.al-shahib@hpa.org.uk