Introduction 61

Chapter 4: Epidemiology

Bernd Sebastian Kamps, Christian Hoffmann

Introduction

Severe acute respiratory syndrome (SARS) is a new infectious disease which was first recognized in late February 2003, when cases of an atypical pneumonia of unknown cause began appearing among staff at a hospital in Hanoi. Within two weeks, similar outbreaks occurred in various hospitals in Hong Kong, Singapore and Toronto.

On March 15, the World Health Organization (WHO) issued emergency travel recommendations to alert health authorities, physicians and the traveling public to what was perceived to be a worldwide threat to health. The travel recommendations marked a turning point in the early course of the SARS outbreak. Areas with cases detected before the recommendations were issued, namely Vietnam, Hong Kong, Singapore and Toronto, experienced the largest and most severe outbreaks, all characterized by chains of secondary transmission outside the healthcare setting. After the recommendations had been issued, all countries with imported cases, with the exception of provinces in China, were able, through prompt detection of cases and isolation of patients, either to prevent further transmission or to keep the number of additional cases very low (WHO. SARS: Status of the Outbreak).

After the disease had moved out of southern China, Hanoi, Hong Kong, Singapore, and Toronto became the initial "hot zones" of SARS, characterized by rapid increases in the number of cases, especially in healthcare workers and their close contacts. In these areas, SARS first took root in hospital settings, where staff, unaware that a new disease had surfaced, exposed themselves to the infectious agent without barrier protection. All of these initial outbreaks were subsequently characterized by chains of secondary transmission outside the healthcare environment (WHO. SARS: Status of the Outbreak).

Now, at the beginning of July, SARS appears to be under control. It might not be all over, though. Toronto, after having had no new cases for more than 20 days, experienced a second outbreak with cases

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62 Epidemiology

linked to at least four hospitals, originating probably from a 96 year old man who had a pneumonia that was misinterpreted as a postoperative complication.

It is probably the "unsuspected SARS patients" that will be a major medical challenge if SARS cannot be eradicated. In Singapore, early in the epidemic (MMWR 52: 405-11), and later in Taiwan (MMWR 52: 461-6), the epidemic was driven partly by cases that either had atypical clinical presentations masking their infections, or were otherwise not rapidly identified because of lack of an initial history of direct contact with a known SARS case – despite efforts to implement extensive control measures. These patients became hidden reservoirs, and the subsequent transmission of the SARS virus resulted in substantial morbidity and mortality and the closure of several large healthcare facilities. Health authorities in Singapore subsequently defined an extended case definition that picked up virtually every person with symptoms that might possibly indicate SARS for investigation and monitoring, regardless of whether the person has been in contact with a SARS patient (see Chapter 5: Prevention).

The number of worldwide cases exceeded 4000 on 23 April and then rapidly soared to 5000 on 28 April, 6000 on 2 May, and 7000 on 8 May, when cases were reported from 30 countries. During the peak of the global outbreak, near the start of May, more than 200 new cases were being reported each day.

As of July 3, 2003, severe acute respiratory syndrome (SARS) had been diagnosed in more than 8,000 patients. The first SARS epidemic can be summarized as follows (Oxford):

1.The epidemiological observation that SARS was first detected in the Guangdong province in November 2002 and took three months to spread even to the immediately neighboring Hong Kong, despite easy exchange of family members between the two areas, does suggest, fortunately, a virus with a low infectiousness.

2.Outbreaks to date have been restricted to families, often living in high-density accommodation, and to hotels and hospitals. This limited spread is the hallmark of a virus with low communicability.

www.SARSreference.com

Modeling the Epidemic 63

3.A truly global respiratory virus like influenza rather quickly emerged to infect millions of persons worldwide. Given the remarkable extent of air travel today, the SARS virus is not spreading rapidly, at least to date.

Modeling the Epidemic

Two major epidemiological studies have been published on the possible consequences of introduction of the SARS virus into a susceptible population (Lipsitch, Riley). Both calculate that the "basic case reproduction number" – the fundamental epidemiological quantity that determines the potential for disease spread – is of the order of 2 to 4 for the Hong Kong epidemic. They draw the conclusion that the SARS coronavirus, if uncontrolled, would infect the majority of people wherever it was introduced, but that it is not so contagious as to be uncontrollable with good, basic public health measures: improved control measures in hospitals, quarantine of contacts of cases, and voluntary reduction in contacts in the population (Dye).

Riley et al. estimate that in Hong Kong, 2.7 secondary infections were generated on average per case at the start of the epidemic, with a substantial contribution from hospital transmission. Transmission rates fell during the epidemic, primarily due to

reductions in population contact rates

improved hospital infection control

more rapid hospital attendance by symptomatic individuals.

Starting Point

In November 2002, cases of a highly contagious and severe atypical pneumonia were noted in the Guangdong Province of southern China. The condition appeared to be particularly prevalent among healthcare workers and members of their household. Many cases were rapidly fatal. During the first week of February there was growing concern among the public about a mysterious respiratory illness, which apparently had a very high mortality and which caused death within hours (Rosling).

Kamps and Hoffmann (eds.)