Current Trends Influenza -- United States, 1983-1984 Season
During the 1983-1984 influenza season, school and college
outbreaks of type A(H1N1) strains began and increased sharply after
January 1 and peaked in February. Type B strains were also
isolated
in all regions of the country from about February to April, largely
in
schools and colleges; a few outbreaks were reported in older age
groups. Type A(H3N2) virus activity was generally sporadic,
despite
an early outbreak in Alaska. Preliminary analysis suggests there
was
little excess mortality associated with influenza.
National data on influenza activity for the 1983-1984 season
were
obtained from four major sources: (1) weekly reports of the number
of
respiratory specimens tested and the number and types of influenza
virus isolates identified by 61 collaborating state, county, city,
or
military laboratories; (2) weekly reports of mortality from 121
cities, an index that has historically reflected seasonal mortality
attributable to influenza; (3) weekly semi-quantitative estimates
from
each state health department of the extent of influenza-like
morbidity
indicated by its individual statewide surveillance system; and (4)
weekly returns from approximately 125 primary-care physician
members
of the American Academy of Family Physicians research panel, who
recorded the number of patients seen in their offices with
influenza-like illnesses* and provided their assessment of whether
an
"outbreak" of influenza was occurring among their patients. In
addition, spontaneous reports of unusual influenza cases and
outbreaks
from a variety of sources were also received by CDC.
Before the usual influenza season, sporadic isolates of
influenza
A(H3N2) virus were obtained in August (Tennessee); of influenza B
virus, in September (Tennessee); and of influenza A(H1N1) virus, in
October (Alabama). The first influenza outbreaks confirmed by
virus
isolation occurred during November and December in Alaska, where
type
A(H3N2) virus was active among young adults. Immediately after the
new year, however, reports of isolations of type A(H1N1) virus from
sporadic cases and outbreaks in schools and colleges were received,
particularly from the South Atlantic and South Central regions.
Rapid
increases in the number of type A(H1N1) virus isolations were noted
(Figure 1) as the virus became active across the country (Figure
2).
Also during November and December, sporadic type B influenza
virus
isolates were recovered in California, Texas, and West Virginia,
primarily from children or young adults. Early in 1984, type B
influenza virus activity increased in association with school
outbreaks and persisted at fairly constant, relatively moderate
levels
from February through April, remaining active into May (Figure 1).
This contrasted with the shorter period of type A(H1N1) virus
activity
that peaked sharply in February. Some nursing-home outbreaks of
type
B influenza were reported. By the end of the season, many regions
had
experienced both type A(H1N1), and type B virus activity, although
early on, type B virus isolates were not identified in the
southeastern states, where type A(H1N1) virus was active in
outbreaks;
conversely, type A(H1N1) isolates were not at first identified in
Washington and Oregon, when type B virus was causing outbreaks
(Figure
2). Influenza A(H3N2) viruses were isolated at low levels
throughout
the season, with at least one associated nursing-home outbreak.
Forty-seven states reported type A(H1N1) isolates; 40 states
reported type B isolates; and 15 states reported type A(H3N2)
isolates. Of the 2,130 isolates obtained by the collaborating
laboratories, 50.6% were influenza type A(H1N1); 44.7% were type B;
and 4.7% were type A(H3N2). The number of isolates reported was
the
largest total since the 1976-1977 season (Figure 3).
Twenty-two state epidemiologists reported widespread influenza
activity during the period of viral circulation in 1983-1984
(Figure
4); in comparison, during the 1981-1982 and 1982-1983 seasons,
widespread activity was reported from only four and 12 states,
respectively. However, during the 1980-1981 season, 32 states
reported widespread activity. Morbidity reports by family
physicians
exhibited temporal and geographic trends consistent with other
indicators of influenza activity. For example, the earliest and
largest rise in visits for influenza-like illness was in the
southern
regions (Figure 5), which corresponded with the large numbers of
type
A(H1N1) outbreaks there.
Preliminary analysis of pneumonia and influenza mortality from
121
cities did not indicate a consistent, statistically significant
rise
above the rates expected in the absence of an epidemic. Failure to
detect excess mortality would correspond with this season's paucity
of
reported outbreaks of influenza among the elderly, the group that
usually is most vulnerable to severe influenza infection.
Reported by State and Territorial Epidemiologists and State
Laboratory
Directors; Other collaborating laboratories; Participating
physicians
of the American Academy of Family Physicians, Statistical Svcs Br,
Div
of Surveillance and Epidemiologic Studies, Epidemiology Program
Office, Computer Systems Office, Office of the Centers Director,
Statistical Svcs Activity, Influenza Br, Div of Viral Diseases,
Center
for Infectious Diseases, CDC.
*Case definition: fever of 37.8 C (100 F) or higher with cough or
sore throat.
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