Hindsight: a re-analysis of the severe acute respiratory syndrome outbreak in Beijing

To review the severe acute respiratory syndrome (SARS) epidemic in Beijing using basic epidemiological principles omitted from the original analysis.

Analysis of Prospective surveillance data for Beijing collected during the outbreak.

Surveillance data were reclassified according to World Health Organization criteria. Cases previously excluded without date of onset of illness were included in the epidemic curve from estimates using the average time between date of onset and date of hospitalization for cases with both dates. Cases who failed to give a contact history were now included; 7% (n=5) of cases during the import phase and 61% (n=365) during the peak phase. Previously excluded cases were included for plotting on an epidemic curve, and basic spot mapping for distribution of cases was used from attack rates recalculated for age, gender, occupation, residential location, date of onset of illness and demographics.

The spot map effectively illustrated clusters by residency, with the inner-city sustaining the highest attack rate (33.42 per 100,000), followed by an easterly distribution 5-30km away (21.62 per 10,000), and lowest in districts 60-160km away (9.21 per 100,000). The new epidemic curve shows the outbreak commencing 10 days earlier than initially reported, with a three-fold greater increase in cases during the escalation phase than previously estimated.

In hindsight, the investigation of the Beijing SARS would have benefited from the use of spot maping as an essential outbreak tool for early identification of specific geographical area(s) for quarantining. If a spot map of incidence density rates was used during the early phase of the outbreak, the inner city might have been identified as a major risk factor requiring rapid quarantining. Contact history became uncommon as the outbreak progressed, suggesting that hospitals were over-burdened or pathogenesis and environment risk factors changed, strengthening the usefulness of early spot mapping and the need to modify risk factors included as contact history as the epidemic progresses.