Risk to a child from Influenza (Flu) if not vaccinated until after the age of 5:
The first live attenuated influenza vaccine was licensed in 2003. Among children 0–4 years of age, hospitalization rates have varied from 100 per 100,000 healthy children to as high as 500 per 100,000 for children with underlying medical conditions. 
Case Fatality Rate: In 2006/07 there were 29 deaths in children under age five and 41 in 2007/08 for an average of 35 flu deaths per year in that age group during that two year period . In 2008 per the US Census the total population of children under 5 in the United States was 21 million .
Rate of Long Term Sequelae or Injury: The most frequent complication of influenza is pneumonia, most commonly secondary bacterial pneumonia (e.g., Streptococcus pneumoniae, Haemophilus influenzae, or Staphylococcus aureus). Primary influenza viral pneumonia is an uncommon complication with a high fatality rate.  If these complications do not result in death, the patient typically does not have permanent injury. Influenza-related encephalitis is rare, occurring at a population incidence of 0.21 per million population.
Incremental Risk in Population with Low Rates of Vaccination (if a child does not vaccinate by age 5):
According to an 2008 appraisal of all comparative studies by The Cochrane Collaboration , the efficacy of the influenza vaccine in children under two was similar to placebo; in other words, the vaccine is not effective in children under two. The researchers concluded that “… at present we could find no convincing evidence that vaccines can reduce mortality, hospital admissions, serious complications and community transmission of influenza” and that “Decision makers’ attention to the vaccination of very young children is not supported by the evidence summarized in our review.”
This analysis is an evidence-based review. Since there is no credible evidence that influenza vaccination reduces death or permanent injuries in children ages 0 – 4, this analysis concludes that the incremental cumulative risk of not vaccinating is zero.
Incremental Risk in Highly Vaccinated Population (if a child does not vaccinate by age 5):
There is no credible evidence of herd immunity due to influenza vaccination. As explained above in “Incremental Risk in Population with Low Rates of Vaccination”, there is no credible evidence that influenza vaccination reduces death or permanent injuries in children ages 0 – 4. Therefore, this analysis concludes that the incremental cumulative risk of not vaccinating is zero.
 Centers for Disease Control and Prevention: Epidemiology and Prevention of Vaccine-Preventable Disease. Atkinson W, Wolfe S, Hamborsky J, McIntyre L. eds. 11th edition. Washington D.C.: Public Health Foundation, 2009.
 Center for Disease Control and Prevention. Notifiable Diseases/Deaths in Selected Cities Weekly Information. MMWR 2009; 58(45): 1276-1287.
 United States Census Bureau. The 2010 Statistical Abstract; Table 78: Live Births, Deaths, Marriages and Divorces: 1960 to 2007.
 Hjalmarsson A, Blomqvist P, Brytting M, Linde A, Skoldenberg B. Encephalitis after influenza in Sweden 1987-1998: a rare complication of a common infection. Eur Neurol. 2009; 61(5):289-94. Epub 2009 Mar 17.
 Smith S, Demicheli V, Di Pietrantonj C, Harnden AR, Jefferson T, Matheson NJ, Rivetti A. Vaccines for preventing influenza in healthy children. Cochrane Database Syst Rev. 2006 Jan 25;(1):CD004879