Routes of Transmission 49

Chapter 3: Transmission

Bernd Sebastian Kamps, Christian Hoffmann

The SARS coronavirus (SARS Co-V) is predominantly spread in droplets that are shed from the respiratory secretions of infected persons. Fecal or airborne transmission seem to be less frequent.

There is growing evidence that a majority of patients might not effectively transmit the virus to other individuals: in Singapore, 162 individuals (81%) of all probable SARS cases had no evidence of transmission of a clinically identifiable illness to other persons (MMWR 52: 405-11). This is in accordance with results from epidemiological studies which indicate that SARS is moderately rather than highly transmissible (Riley).

In some instances, however, so-called "superspreader" patients are able to transmit the SARS virus to a large number of individuals. Superspreaders and nosocomial amplification were the driving factors behind the early 2003 outbreaks.

Routes of Transmission

The fact that the majority of new infections occurred in close contacts of patients, such as household members, healthcare workers, or other patients who were not protected with contact or respiratory precautions, indicates that the virus is predominantly spread by droplets or by direct and indirect contact (CDC, Seto).

The presence of virus in the stool suggests the possibility of oral-fecal transmission (Drosten, Peiris 2003b). This is reminiscent of characteristics of other coronaviruses (Cho), and feces are therefore potentially an additional route of transmission. In the Amoy Gardens outbreak (see Chapter 4: Epidemiology, Hong Kong), the SARS virus may have been spread through the sewage systems of the buildings (for details, see Government of Hong Kong Special Administrative Region).

Kamps and Hoffmann (eds.)

50 Transmission

The airborne spread of SARS does not seem to be a major route of transmission. However, the apparent ease of transmission in some instances is of concern. In particular, the cases in the original Hong Kong cluster that originated at the Metropole hotel (MMWR 52:241- 8) and in the Amoy Gardens Outbreak (Government of Hong Kong Special Administrative Region) indicate that the possibility of airborne transmission of the SARS virus, although probably a rare event, cannot be ruled out. Clusters among healthcare workers exposed during high-risk activities (i.e., endotracheal intubation, bronchoscopy, sputum induction) seem to confirm airborne transmission via a contaminated environment (i.e., re-aerosolization when removing protective equipment, etc.)

There are currently no indications that any goods, products or animals arriving from areas with SARS outbreaks pose a risk to public health. The WHO does not recommend any restrictions in this regard (http://www.who.int/csr/sars/goods2003_04_10).

Factors Influencing Transmission

Whether the transmission of a viral pathogen leads to the manifestation of the disease is determined by the intricate interplay of a multitude of still largely undefined viral and host factors.

As in other infectious diseases, the size of the inoculum, i.e., the number of infectious particles that are transmitted from one person to another, is probably of major importance. The size of the inoculum is determined by

the viral load in the secretion of the index patient, and

the distance to the index patient (face-to-face contact, crowded locations, i.e., a sneeze in the elevator)

Surprisingly, in the first few days after the onset of SARS-related symptoms, the amount of virus detected in secretion from the respiratory tract seems to be relatively low. Findings from sequential quantitative RT-PCR analyses of nasopharyngeal aspirates suggest that the viral load might peak only at around day 10 after the onset of symptoms and then decrease to the levels obtained on admission at day 15

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