Measuring underreporting and under-ascertainment in infectious disease datasets: a comparison of methods

TLDR

Efficient surveillance systems are essential for public health, yet they are hampered by under‑ascertainment and underreporting, which create uncertainty about true disease incidence. This review examines methods for estimating under‑ascertainment and underreporting of salmonellosis and campylobacteriosis across EU, EFTA, and selected OECD countries within the ECDC‑funded BCoDE project. The authors extracted and compared multiplication factors from the literature, deriving them where necessary, to evaluate how underestimation varies by pathogen, country, age, and sex. The findings demonstrate that multiplication factors differ markedly across diseases and jurisdictions, highlighting the importance of selecting appropriate, disease‑specific factors to adjust surveillance data and yield more realistic incidence estimates.

Abstract

Efficient and reliable surveillance and notification systems are vital for monitoring public health and disease outbreaks. However, most surveillance and notification systems are affected by a degree of underestimation (UE) and therefore uncertainty surrounds the 'true' incidence of disease affecting morbidity and mortality rates. Surveillance systems fail to capture cases at two distinct levels of the surveillance pyramid: from the community since not all cases seek healthcare (under-ascertainment), and at the healthcare-level, representing a failure to adequately report symptomatic cases that have sought medical advice (underreporting). There are several methods to estimate the extent of under-ascertainment and underreporting.Within the context of the ECDC-funded Burden of Communicable Diseases in Europe (BCoDE)-project, an extensive literature review was conducted to identify studies that estimate ascertainment or reporting rates for salmonellosis and campylobacteriosis in European Union Member States (MS) plus European Free Trade Area (EFTA) countries Iceland, Norway and Switzerland and four other OECD countries (USA, Canada, Australia and Japan). Multiplication factors (MFs), a measure of the magnitude of underestimation, were taken directly from the literature or derived (where the proportion of underestimated, under-ascertained, or underreported cases was known) and compared for the two pathogens.MFs varied between and within diseases and countries, representing a need to carefully select the most appropriate MFs and methods for calculating them. The most appropriate MFs are often disease-, country-, age-, and sex-specific.When routine data are used to make decisions on resource allocation or to estimate epidemiological parameters in populations, it becomes important to understand when, where and to what extent these data represent the true picture of disease, and in some instances (such as priority setting) it is necessary to adjust for underestimation. MFs can be used to adjust notification and surveillance data to provide more realistic estimates of incidence.