EPIDEMIOLOGY OF A SMALL PERTUSSIS OUTBREAK IN KENT COUNTY, MICHIGAN.

on the morbidity and mortality of the particu¬ lar disease. It is therefore of special signifi¬ cance to observe the status of whooping cough in Kent County, Mich., including the city of Grand Rapids, a community where field trials of pertussis vaccine were conducted over a pe¬ riod of years (i, #), and where immunization has been maintained at a relatively high level for a long time (3). In addition, there has been continuous use of culture methods in the diag¬ nosis of whooping cough for more than 30 years. In 1960, Kent County including Grand Rapids had a population of approximately 363,000, while for the city alone the figure was slightly less than 200,000. Data on the trend of whooping cough mor¬ tality in the United States, Michigan, and Grand Rapids are given in table 1. Mortality rates had decreased markedly before the use of pertussis vaccine was sufficiently widespread to account for the decline in incidence. The de¬ crease has been more rapid since 1940. Some observations on a shift in age group of those experiencing the disease are also of in¬ terest. As early as 1955 Ipsen and Bowen (4) noted that in Massachusetts the attack rate of pertussis began failing around 1943 among chil¬ dren under 10, while it had not yet changed among those in the 10-14 age group. These authors suggested that the use of pertussis vac¬ cine had only recently been sufficiently wide-