Pertussis: herd immunity and vaccination coverage in St Lucia

Abstract

In a single complete epidemic in St Lucia, an island too small to support constant clinical pertussis, the pertussis case rates in small communities (villages and small towns) with differing levels of vaccination coverage of young children were compared. The association between greater vaccination coverage and greater herd immunity was clear, despite the imperfect protection given to individuals. An analysis in terms of population dynamics is evidence against the theory that endemic subclinical pertussis maintains transmission in a highly vaccinated population. We suggest that with a homogeneous vaccination coverage of 80% of 2-year-old children pertussis might be eradicated from the island, and that this is a practicable experiment.

PIP: Pertussis case rates in small communities of St Lucia with differing levels of vaccination coverage were compared during a pertussis epidemic in 1981. St Lucia has experienced epidemics of whooping cough at 2-5 year intervals, but no sporadic cases have been reported between cycles. By 1981, under 30% of 2 year olds on the island had received fewer than 3 DPT injections compared with 60% in 1976. The number of reported cases of whooping cough declined from 921 in 1975-76 to 470 in 1980-81. The mean age at infection in the 1981 epidemic was 4.43 years. There was a significant association between greater vaccination coverage and greater herd immunity against clinical pertussis. Vaccination coverage by health center ranged from a low of 35.2% to a high of 92.6%. The number of cases of pertussis/1000 1-3 year olds ranged from a low of zero to a high of 388 at a center where immunization coverage was 71.5%. Of the 352 cases of pertussis occurring in 1981 that involved children 1-3 years of age, 230 were from health center areas with vaccination coverage levels below 80% while only 122 cases were reported from areas with coverage rates over 80%. It is suggested that a homogeneous coverage rate of 80% of 2 year olds would eradicate pertussis from St Lucia.