Chains of Measles Transmission -- United States, 1982
In 1982, a provisional total of 1,697 cases of measles was
reported to CDC, a record low incidence rate of 0.7 cases per
100,000
population for all ages. Fifteen states reported no measles cases
all
year, and an additional seven states reported only imported cases.
Ninety-four percent of the nation's 3,138 counties reported no
measles
all year.
October 1, 1982, was the target date for eliminating indigenous
transmission of measles. To investigate the remaining chains of
transmission, CDC has reviewed reporting forms from 556 measles
cases,
33% of the 1982 total, submitted by 11 of the 28 states reporting
cases. Each form was reviewed for patient age, immunity status,
and
day-care-center attendance. Cases were grouped using CDC's measles
classification system (Table 1) (1). Of 556 persons with measles,
209
(37.6%) were preschoolers (less than 5 years old), and 281 (50.5%)
were school-aged (5-19 years old). Overall, 337 (60.6%) cases were
not preventable because the patients were either too young or too
old
for routine vaccination or because they had evidence of immunity.
Of
the 219 (39.4%) preventable cases, 109 occurred among children who
attended schools or day-care centers and were thus readily
accessible
to control measures. The other 110 were not readily
accessible--not
school-aged and not known to attend a day-care center.
CDC has also reviewed detailed, written reports of the 14 major
outbreaks in 1982. Of 1,697 measles cases, 63% occurred in 14
separate outbreaks or chains of transmission (defined as consisting
of
two or more generations).* The other 37% occurred sporadically.
The
14 outbreaks were reported from nine states and ranged in size from
nine to 419 cases in two to 16 generations (Figure 2). Most were
of
short duration. Sources were identified for 11 of the 14
outbreaks:
eight were foreign importations, two were out-of-state
importations,
and one was indigenous from a child with a medical exemption. In
eight outbreaks for which data were available, 42% of subsequent
first-generation cases were preventable. At least six of the 14
chains of transmission could have been prevented because the index
cases were imported in persons who had not been appropriately
vaccinated. In contrast, none of the outbreaks had an indigenous,
preventable source. In eight outbreaks for which data were
available,
the duration of the outbreak was directly related to the proportion
of
preventable first-generation cases (r=0.95) (Figure 3).
Provisional data for the first quarter of 1983 show a 2-fold
increase in measles morbidity over the same period in 1982;
however,
the chains of transmission have been limited. Approximately 85% of
the cases reported have occurred in nine discrete outbreaks, and
53%
of the cases reported have occurred among college and university
students. Ninety-nine percent of counties reported no measles
during
the first quarter of 1983.
Reported by Div of Immunization, Center for Prevention Services,
CDC.
Editorial Note
Editorial Note: Failure to vaccinate--rather than vaccine
failure--was
the major risk factor for sustained transmission in 1982 (2).
Unvaccinated susceptibles were important in sustaining measles
transmission. The outbreaks with the greatest proportion of
preventable cases in the first generation after the index case
continued for the longest period of time, suggesting that the
overall
immunity levels were lower in areas with high proportions of
preventable first-generation cases. The outbreaks were sustained
because enough preventable cases, in unvaccinated susceptibles,
joined
with vaccine failures to increase the total supply of susceptibles.
Although measles in 1983 has affected less than three per
100,000
college students and less than 1% of college campuses, campus
outbreaks have focused attention on susceptibility among college
students (3). Colleges can help remedy this situation by
establishing
vaccination requirements for all students born after 1956. In
addition, efforts should be made to identify and immunize remaining
susceptibles in high schools, especially those in eleventh and
twelfth
grades.
Indigenous measles has been eliminated from most of the United
States. The remaining chains of transmission are limited in size
and
extent. Analysis of the chains demonstrates that the measles
elimination strategy is valid. The major emphasis must now be
placed
on improving vaccination among preschoolers and college students,
while continually enforcing school requirements, and on
aggressively
responding to the remaining chains of transmission.
References
CDC. Classification of measles cases and categorization of
measles
elimination programs. MMWR 1983;31:707-11.
Amler RW, Bloch AB, Orenstein WA, et al. Measles in the United
States: chains of transmission. Proceedings of the 18th
Immunization Conference, May 16-19, 1982. Atlanta: CDC (in
press).
CDC. Measles outbreaks on university campuses--Indiana, Ohio,
Texas. MMWR 1983;32:193-5.
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