In conclusion, S-OIV could stimulate the cellular immune response. When accompanied by CD19 + B cell increase and a continuous NK cell decline, it could indicate that the body is in a dynamic balance of the antiinfection immunity and autoimmune damage; this is particularly true for patients with severe influenza. Each lymphocyte subgroup in patients with H1N1 plays a more important antiviral role in the early stages of disease, but excessive immune response also leads to the increase and development of infection. Maternal Influenza Immunization and Reduced Likelihood of Prematurity and Small for Gestational Age Births: A Retrospective Cohort Study BACKGROUND: Infections during pregnancy have the potential to adversely impact birth outcomes. We evaluated the association between receipt of inactivated influenza vaccine during pregnancy and prematurity and small for gestational age (SGA) births. METHODS AND FINDINGS: We conducted a cohort analysis of surveillance data from the Georgia (United States) Pregnancy Risk Assessment Monitoring System. Among 4,326 live births between 1 June 2004 and 30 September 2006, maternal influenza vaccine information was available for 4,168 (96.3%). The primary intervention evaluated in this study was receipt of influenza vaccine during any trimester of pregnancy. The main outcome measures were prematurity (gestational age at birth <37 wk) and SGA (birth weight <10th percentile for gestational age). Infants who were born during the putative influenza season (1 October–31 May) and whose mothers were vaccinated against influenza during pregnancy were less likely to be premature compared to infants of unvaccinated mothers born in the same period (adjusted odds ratio [OR] = 0.60; 95% CI, 0.38–0.94). The magnitude of association between maternal influenza vaccine receipt and reduced likelihood of prematurity increased during the period of at least local influenza activity (adjusted OR = 0.44; 95% CI, 0.26–0.73) and was greatest during the widespread influenza activity period (adjusted OR = 0.28; 95% CI, 0.11–0.74). Compared with newborns of unvaccinated women, newborns of vaccinated mothers had 69% lower odds of being SGA (adjusted OR = 0.31; 95% CI, 0.13–0.75) during the period of widespread influenza activity. The adjusted and unadjusted ORs were not significant for the pre-influenza activity period. CONCLUSIONS: This study demonstrates an association between immunization with the inactivated influenza vaccine during pregnancy and reduced likelihood of prematurity during local, regional, and widespread influenza activity periods. However, no associations were found for the pre-influenza activity period. Moreover, during the period of widespread influenza activity there was an association between maternal receipt of influenza vaccine and reduced likelihood of SGA birth. Please see later in the article for the Editors' Summary Infections during pregnancy have the potential to adversely impact birth outcomes and fetal growth and development. Respiratory infections-particularly those resulting in pneumonia-have been associated with low birth weight and increased risk of preterm birth [1, 2] . Influenza virus is an important respiratory pathogen that causes substantial burden of diseaseincluding morbidity and mortality among pregnant women, with greater risk of influenza-related morbidity among pregnant women than among non-pregnant and postpartum women [3] .