Epidemiological characterization of a fourth wave of pandemic A/H1N1 influenza in Mexico, winter 2011-2012: age shift and severity.
OBJECTIVE
A substantial recrudescent wave of pandemic influenza A/H1N1 affected the Mexican population from December 1, 2011-March 20, 2012 following a 2-year period of sporadic transmission.


METHODS
We analyzed demographic and geographic data on all hospitalizations with severe acute respiratory infection (SARI) and laboratory-confirmed A/H1N1 influenza, and inpatient deaths, from a large prospective surveillance system maintained by a Mexican social security medical system during April 1, 2009-March 20, 2012. We also estimated the reproduction number (R) based on the growth rate of the daily case incidence by date of symptoms onset.


RESULTS
A total of 7569 SARI hospitalizations and 443 in-patient deaths (5.9%) were reported between December 1, 2011, and March 20, 2012 (1115 A/H1N1-positive inpatients and 154 A/H1N1-positive deaths). The proportion of laboratory-confirmed A/H1N1 hospitalizations and deaths was higher among subjects ≥60 years of age (χ(2) test, p <0.0001) and lower among younger age groups (χ(2) test, p <0.04) for the 2011-2012 pandemic wave compared to the earlier waves in 2009. The reproduction number of the winter 2011-2012 wave in central Mexico was estimated at 1.2-1.3, similar to that reported for the fall 2009 wave, but lower than that of spring 2009.


CONCLUSIONS
We documented a substantial increase in the number of SARI hospitalizations during the period December 2011-March 2012 and an older age distribution of laboratory-confirmed A/H1N1 influenza hospitalizations and deaths relative to 2009 A/H1N1 pandemic patterns. The gradual change in the age distribution of A/H1N1 infections in the post-pandemic period is consistent with a build-up of immunity among younger populations.
Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Instituto Mexicano del Seguro Social, Mexico, DF, Mexico.
Víctor H
VH
Borja-Aburto
Gerardo
G
Chowell
Cécile
C
Viboud
Lone
L
Simonsen
Mark A
MA
Miller
Concepción
C
Grajales-Muñiz
Cesar R
CR
González-Bonilla
Jose A
JA
Diaz-Quiñonez
Santiago
S
Echevarría-Zuno
D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D050936:Basic Reproduction Number; D002675:Child, Preschool; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification / Q000472:pathogenicity*; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000635:transmission / Q000821:virology*; D007297:Inpatients / Q000706:statistics & numerical data; D008297:Male; D008800:Mexico / Q000453:epidemiology; D008875:Middle Aged; D058873:Pandemics / Q000706:statistics & numerical data*; D012621:Seasons; D055815:Young Adult