Prevalence of 2009 pandemic influenza A (H1N1) virus antibodies, Tampa Bay Florida--November-December, 2009.
BACKGROUND
In 2009, a novel influenza virus (2009 pandemic influenza A (H1N1) virus (pH1N1)) caused significant disease in the United States. Most states, including Florida, experienced a large fall wave of disease from September through November, after which disease activity decreased substantially. We determined the prevalence of antibodies due to the pH1N1 virus in Florida after influenza activity had peaked and estimated the proportion of the population infected with pH1N1 virus during the pandemic.


METHODS
During November-December 2009, we collected leftover serum from a blood bank, a pediatric children's hospital and a pediatric outpatient clinic in Tampa Bay Florida. Serum was tested for pH1N1 virus antibodies using the hemagglutination-inhibition (HI) assay. HI titers ≥40 were considered seropositive. We adjusted seroprevalence results to account for previously established HI assay specificity and sensitivity and employed a simple statistical model to estimate the proportion of seropositivity due to pH1N1 virus infection and vaccination.


RESULTS
During the study time period, the overall seroprevalence in Tampa Bay, Florida was 25%, increasing to 30% after adjusting for HI assay sensitivity and specificity. We estimated that 5.9% of the population had vaccine-induced seropositivity while 25% had seropositivity secondary to pH1N1 virus infection. The highest cumulative incidence of pH1N1 virus infection was among children aged 5-17 years (53%) and young adults aged 18-24 years (47%), while adults aged ≥50 years had the lowest cumulative incidence (11-13%) of pH1N1 virus infection.


CONCLUSIONS
After the peak of the fall wave of the pandemic, an estimated one quarter of the Tampa Bay population had been infected with the pH1N1 virus. Consistent with epidemiologic trends observed during the pandemic, the highest burdens of disease were among school-aged children and young adults.
Epidemic Intelligence Service, Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, United States of America. cyv5@cdc.gov
Chad M
CM
Cox
Kate
K
Goodin
Emily
E
Fisher
Fatimah S
FS
Dawood
Janet J
JJ
Hamilton
German F
GF
Leparc
Monica
M
Gray
Linda
L
Nelson
Rebekah H
RH
Borse
James A
JA
Singleton
Carrie
C
Reed
Amanda L
AL
Balish
Jacqueline M
JM
Katz
Richard S
RS
Hopkins
Alicia M
AM
Fry
D000293:Adolescent; D000328:Adult; D000914:Antibodies, Viral / Q000097:blood*; D002648:Child; D002675:Child, Preschool; D005431:Florida / Q000453:epidemiology; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology* / Q000472:pathogenicity*; D007251:Influenza, Human / Q000097:blood* / Q000453:epidemiology*; D008875:Middle Aged; D058873:Pandemics; D012621:Seasons; D016036:Seroepidemiologic Studies; D055815:Young Adult
D000914:Antibodies, Viral