In 1983, the reported occurrence of measles reached its lowest
level since national reporting of measles began in 1912. A
provisional total of 1,436 cases was reported, for a record low
incidence rate of 0.6 cases per 100,000 population for all ages
(Figure 1). This is a 99.7% reduction from the prevaccine era,
when,
from 1950 to 1962, an annual average of 525,730 cases was reported
(315.2 cases/100,000 population), and a 16.2% reduction from the
1,714
cases reported in 1982, the previous year of record low incidence
(0.7/100,000) (1). Fewer than 100 indigenous cases* were reported
each week in 1983, except week 12, when 138 such cases were
reported.
Fewer than 50 such cases were reported during 45 weeks, and fewer
than
10 were reported during 22 weeks.
Of 1,136 indigenous cases, 877 (77.2%) were reported from four
states--Indiana (402), Illinois (173), California (153), and
Florida
(149). Most other areas reported few or no measles cases (Figure
2).
Twenty-six states and the District of Columbia reported no
indigenous
cases all year, compared to 22 states in 1982. Of the remaining 24
states, 20 states and New York City reported fewer than 50
indigenous
cases, and 15 states reported fewer than 10 such cases. Of the
nation's 3,139 counties, 3,002 (95.6%) reported no measles cases
during the entire year, compared to 2,944 (93.8%) in 1982. Every
county was free of reported measles for at least 6 consecutive
weeks
in 1983. Only six counties (0.2%) reported measles during the last
4
weeks of 1983. The three chains of transmission that occurred
during
the last 2 months of the year were concentrated in certain
subpopulations that either refused immunization for religious or
philosophic reasons or did not routinely seek medical care or
immunization (3).
Most transmission in 1983 was concentrated in settings other
than
primary or secondary schools. Of 31 discrete chains of
transmission**
involving 1,233 cases, five chains accounted for 62.7% (900/1,436)
of
all cases reported in 1983. All five occurred primarily in
settings
other than primary or secondary schools. The largest chain, which
accounted for 32.4% (465/1,436) of all cases in 1983, was
concentrated
among college students (4). Three additional chains (362 cases)
occurred primarily among children less than 5 years old (5). The
fifth chain (73 cases) involved schoolchildren and other age
groups,
but most transmission occurred when schools were not in session.
Eleven of the 26 smaller chains of transmission were concentrated
in
groups other than schoolchildren. The remaining 15 smaller chains
(209 cases) principally or partially involved schoolchildren. Of
these, 10 chains (90 cases) lasted two to three generations, and
five
chains (119 cases) lasted four to seven generations. Thus, chains
of
transmission in primary and secondary schools accounted for only
14.6%
(209/1,436) of reported cases in 1983.
According to detailed information received by CDC's Division of
Immunization, international importations and associated cases
together
accounted for 14.7% (211/1,436) of all measles cases reported in
1983. Ninety (6.3%) cases were international importations,* with
sources from 30 different countries--an average of 1.7
international
importations per week. In addition, 121 cases were
epidemiologically
linked to 26 international importations within two generations of
infection.
Reported by Div of Immunization, Center for Prevention Svcs, CDC.
Editorial Note
Editorial Note: The target date for measles elimination was
October
1, 1982. Although elimination has not yet been achieved, these
provisional data demonstrate substantial changes in the
epidemiology
of measles in the United States in 1983. The lack of measles
transmission in over 95% of counties and the lack of reported
incidence in every county for 6 or more weeks indicate that
transmission was at least temporarily interrupted in every
community
in the United States during 1983. The data also suggest that less
than 15% of documented measles transmission in 1983 involved
primary
and secondary schools, reflecting the dramatic success of state
school
immunization requirements in raising measles immunization levels
and
lowering measles incidence rates in schoolchildren. In the
1982-1983
school year, immunity to measles was documented*** for 97% of
children
in the United States entering school for the first time.
Nevertheless, several important challenges to elimination
remain.
Four of the five largest outbreaks occurred primarily among
preschoolers under 5 years old and college students who are not
directly affected by state immunization requirements. Outbreaks
among
preschoolers were not concentrated in licensed day-care centers,
where
immunization requirements have been implemented by 45 states and
the
District of Columbia. Outbreaks among college and university
students
indicate that sufficient numbers of susceptibles may exist to
sustain
transmission when measles is introduced. Many may have missed
needed
vaccinations or may have escaped natural infection because of
decreasing transmission during their childhood. Outbreaks on
campuses
were difficult to control because susceptible students could not be
identified readily. Few campuses required or systematically
recorded
documentation of immunity. In the future, measles cases among
young
adults may be difficult to identify because physicians who commonly
treat this group may not consider measles as a likely diagnosis
(6).
The exact date of the elimination of indigenous measles
transmission from the United States will only be known in
retrospect
(7). The available data demonstrate that the measles elimination
strategy--achievement and maintenance of high immunization levels,
development of strong and effective surveillance systems, and
aggressive response to the occurrence of disease--remains valid.
Although it is not clear how important preschool- and college-aged
populations are in sustaining transmission independently, recent
outbreaks among these groups suggest that additional efforts may be
required to implement the elements of the strategy in these groups
before measles is completely eliminated in the United States.
References
Amler RW, Bloch AB, Orenstein WA, et al. Measles in the United
States: chains of transmission. 18th Immunization Conference
Proceedings, Atlanta, Georgia, CDC, May 16-19, 1983, pp. 17-21.
CDC. Classification of measles cases and categorization of
measles elimination programs. MMWR 1982;31:707-11.
CDC. Interstate transmission of measles in a Gypsy
population--Washington, Idaho, Montana, California. MMWR
1983;32:659-62.
Amler RW, Kim-Farley RF, Orenstein WA, et al. Measles on
campus.
J Am College Health 1983;32:53-7.
CDC. Measles among children of migrant workers--Florida. MMWR
1983;32:471-2, 477-8.
Amler RW, Orenstein WA. Measles in young adults. Postgraduate
Med
(in press).
CDC. Elimination of indigenous measles--United States. MMWR
1982;31:517-9.
*Defined in CDC measles case classification (2).
**Chain of transmission defined as a series of measles cases
consisting of an index case followed by at least two generations of
epidemiologically-linked cases.
***Written documentation of date of administration of live measles
vaccine on or after the first birthday or of physician-diagnosed
measles illness.
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