Force of Infection (FOI)

Force of infection (FOI) is a term used in infectious disease epidemiology to describe the rate at which susceptible individuals in a population acquire an infection.

It reflects the per-person risk of becoming infected over a given period, typically expressed as a rate per year.

FOI captures both the biological transmissibility of a pathogen and the intensity of exposure in a population.

Why force of infection is important

Unlike the basic reproduction number (R₀), which assumes a fully susceptible population, FOI can change over time as population immunity increases due to infection or vaccination.

How FOI differs from other transmission metrics

It’s especially useful in modeling endemic diseases or in evaluating how transmission varies by age, geography, or behavioral risk factors.

Methods for estimating FOI

• Age-specific seroprevalence surveys
• Mathematical modeling based on case data
• Longitudinal cohort studies
• Cross-sectional antibody testing
• Use of proxy exposure indicators (e.g., urban vs. rural)
• Historical infection trend analysis

Settings where FOI is commonly applied

• Endemic disease surveillance
• Vaccine impact evaluations
• Outbreak risk assessments
• Zoonotic spillover modeling
• Age-targeted immunization planning

These applications make FOI a valuable tool in both research and real-time public health decision-making.

Diseases commonly studied using FOI

• Hepatitis A
• Toxoplasmosis
• Cytomegalovirus (CMV)

Using FOI to guide public health action

• Target vaccination to high-incidence age groups
• Identify geographic hotspots for intervention
• Assess long-term impact of improved sanitation

Case study: FOI and hepatitis A

High early-life FOI in low-resource settings

In parts of South Asia and sub-Saharan Africa, the FOI for hepatitis A virus is very high in early childhood.

• Most children infected by age 5
• Infections often asymptomatic or mild
• Leads to widespread early immunity

Changing FOI with socioeconomic development

Improved sanitation can delay infection to older ages, increasing disease severity and changing outbreak risk.

• More adolescents and adults remain susceptible
• Outbreaks more likely to cause severe illness
• Targeted vaccination becomes more important

FOI insights from recent U.S. outbreaks

Hepatitis A outbreaks among men who have sex with men or people experiencing homelessness show the role of FOI in shaping response.

• Clusters often in high-risk social networks
• Vaccination campaigns tailored to affected groups
• Enhanced surveillance in urban public health departments

Applying FOI to global vaccine strategies

Estimating FOI helps determine the age and frequency of vaccine doses needed to sustain population immunity.

• Supports age-structured immunization schedules
• Guides decisions on booster doses
• Anticipates shifts in immunity over time

FOI in the context of asymptomatic infections

Seroprevalence data help estimate FOI for diseases that go undiagnosed due to lack of symptoms.

FOI as a planning and evaluation tool

• Model the long-term impact of health interventions
• Predict consequences of population aging or urbanization
• Support economic evaluations of vaccine programs

Future directions for FOI analysis

Improving real-time and predictive applications

• Integrate FOI into outbreak forecasting models
• Use geospatial data to refine local estimates
• Link FOI trends with climate and migration data
• Develop interactive tools for health officials
• Expand serological surveillance in underserved regions