Expected Influenza Infections by Age 30 in Unvaccinated Individuals
An unvaccinated person in the United States will likely contract influenza approximately 5-10 times by age 30, though precise epidemiologic data quantifying lifetime infection frequency in unvaccinated cohorts is not directly reported in current guidelines.
Why This Estimate Is Reasonable
The available evidence provides indirect support for this estimate through several key observations:
Children have the highest infection rates across all age groups, with rates of infection being highest among children even though serious illness and death rates are highest among elderly persons 1
School-aged children bear a large influenza disease burden and have significantly higher chances of seeking influenza-related medical care compared with healthy adults 1
Influenza epidemics occur nearly every winter in the United States, creating annual exposure opportunities 1
Young children aged 0-23 months face hospitalization rates of 1,040 per 100,000 population, comparable to elderly adults, indicating substantial infection burden early in life 2
Age-Specific Infection Patterns
Childhood (Birth to Age 18)
- Children experience the highest rates of influenza infection, with exposure amplified by group settings in childcare and schools 1
- Children younger than 2 years are at elevated risk of hospitalization and complications, suggesting frequent infections during early childhood 1
- Most infections during childhood years would occur during this 18-year window, likely accounting for 3-6 infections
Young Adulthood (Ages 18-30)
- Healthy adults aged 18-49 years have mortality rates of only 0.4-0.6 per 100,000, indicating most infections are mild 2
- Despite lower mortality, influenza still causes substantial morbidity, healthcare visits, and work absenteeism in this age group 1
- An additional 2-4 infections would be expected during this 12-year period
Critical Limitations of Available Data
The medical literature focuses almost exclusively on vaccination effectiveness, hospitalization rates, and mortality rather than lifetime infection frequency in unvaccinated populations. The guidelines reviewed 1, 3 emphasize vaccination recommendations and outcomes but do not provide direct epidemiologic surveillance data on cumulative infection rates by age in unvaccinated cohorts.
Why Vaccination Matters for This Population
- Vaccination reduces outpatient influenza illness by approximately 50-75% in most people when vaccine strains match circulating viruses 1
- Even in seasons with poor vaccine match (such as 2014-15 with only 23% effectiveness or 2021-22 with 16% effectiveness), vaccination still prevents some infections and serious complications 4, 5
- Recent 2023-24 season data show vaccine effectiveness of 59-67% in children and 33-49% in adults against outpatient visits 6
Common Pitfalls in Interpreting Infection Risk
- Not all influenza infections are symptomatic or medically attended, so the true infection rate is higher than what healthcare utilization data suggest 1
- Infection rates vary dramatically by season depending on predominant circulating strain, with H3N2 seasons causing higher morbidity than H1N1 or influenza B seasons 1, 2
- Prior infection provides some strain-specific immunity, but antigenic drift means protection wanes and new infections remain possible in subsequent seasons 1