UK IID2 Study: Tam et al. (2012) Burden
A Million GP Visits: The UK’s Definitive Gut-Disease Study
The study: Tam CC, Rodrigues LC, Viviani L, et al. (2012). Longitudinal study of infectious intestinal disease in the UK (IID2 study): incidence in the community and presenting to general practice. Gut, 61(1), 69–77.
How much intestinal disease is really out there? The UK’s IID2 study — a massive prospective cohort with nested GP surveillance — measured it directly: community incidence, GP consultation rates, and the underreporting multipliers. It’s the gold standard for burden methodology.
What IID2 found
Around 17 million community cases yearly (roughly 1 in 4 people), with about 1 million GP consultations — the underreporting pyramid quantified: for every case reaching national surveillance, ~147 occurred in the community (varying by pathogen). Norovirus and Campylobacter dominated; the foodborne fraction was estimated separately.
The methodological contribution
IID2’s prospective design (following a cohort over time, capturing cases that never reach healthcare) is the most rigorous burden method — superior to multiplier models built on surveillance alone. The study validated the multiplier approach while providing better multipliers.
The implications
The burden is vastly larger than surveillance suggests — policy based on reported cases alone misses ~99% of community illness. For food safety: the foodborne fraction of this burden is the target, and IID2’s methods show how to measure it. For other countries: IID2 is the template — the UK invested in getting the number right, and the investment pays off in better-targeted interventions. The study’s lasting message: measure the community, not just the clinic. The iceberg is almost entirely underwater.
The community burden, measured directly
The IID2 study — Tam et al.’s 2012 longitudinal study of infectious intestinal disease in the UK — measured the community burden directly: the prospective cohort (the participants reporting the symptoms weekly), the incidence estimated (the 274 cases per 1,000 person-years — the one-in-four annual risk), the general-practice presentation (the fraction seeking care), the pathogen attribution (the norovirus leading, the Campylobacter leading the bacterial). The study’s design (the community cohort, not just the surveillance) captured the pyramid’s base.
The study’s multiplier findings: the community-to-surveillance ratios (the hundreds of community cases per reported case, varying by pathogen), the underreporting quantified directly rather than estimated. The IID2’s numbers (the 147 community cases per reported Salmonella, the 137 per Campylobacter — the pathogen-specific multipliers) gave the burden estimators the empirical foundation.
The study’s policy impact: the UK food safety strategy informed by the true burden (the norovirus and the Campylobacter priorities), the surveillance interpretation (the reported trends read against the community multipliers), the international influence (the IID2 as the model community study). The 2012 paper’s message: the surveillance sees the tip; the community study measures the iceberg; and the food safety priorities should reflect the whole.
The IID2’s methodological advance
The IID2’s methodological advance — the prospective cohort, the microbiological testing, the general-practice surveillance, the triangulation — over the IID1 set the standard the subsequent burden studies emulated: the multiple methods, the underreporting quantified, the pathogen-specific estimates. The study’s design (the expensive, the comprehensive) is the burden-estimation gold standard. The countries that replicated it (the adapted to the local systems) got the comparable numbers.
Source: Tam CC et al. (2012). Longitudinal study of infectious intestinal disease in the UK (IID2 study). Gut, 61(1), 69–77.