A person wearing a protective face mask walks past a temporarily closed pub named after the founding father of epidemiology John Snow. Photo / Leon Neal, Getty Images
A person wearing a protective face mask walks past a temporarily closed pub named after the founding father of epidemiology John Snow. Photo / Leon Neal, Getty Images
COMMENT
An acute Covid outbreak occurs when two or more confirmed cases link to the same setting. And in the week ending the June 28, 43 were reported in English workplaces.
The identification and sizing of these outbreaks are due to the wider availability of testing and therefore, better detection.
Outbreak investigation, however, provides a unique opportunity to study the dynamics of viral transmission in a defined population (factories, ships, aircraft, dormitories etc). What accelerates or delays its spread, what is the role of asymptomatic infected people, how does the workplace promote an outbreak?
These questions (the classic what? where? and when? of epidemiology) were understood by John Snow who in 1854 investigated the cholera outbreak in London.
Snow recognised the significance of outliers in explaining his theory that cholera was transmitted via the oro-faecal route from the contaminated water of the Broad Street pump. The vast majority of his colleagues were convinced that cholera and infectious diseases originated from miasmata or bad smells from rotting rubbish and other dirt.
The widow of Hampstead was one such outlier. She lived more than 6km from the pump - surely too far to drink the contaminated water. Snow's in-depth field investigations revealed that water was taken from the pump and transported to Hampstead where it was drunk by the widow two days before her death - thus confirming his hypothesis.
Snow prevailed on the parish council which may have removed the pump handle from the Broad Street pump but more importantly ordered an excavation of the well and its contiguous cesspit and slowed leakage, making the mixing of dirty and clean water possible.
The story shows that the when, where and who of infectious outbreaks needs describing meticulously and hypotheses formulating based on the evidence and then testing. The importance of such investigations cannot be overstated. We now have several outbreaks - some in similar settings. The critical question is: what connects them?
Investigations into the 2003 Sars outbreak in Hong Kong revealed the infection was accelerated by oro-faecal spread from toilet aerosol in the Amoy Gardens block of flats. The design of the bathroom floors meant the toilet traps ran dry which allowed viral aerosols into the bathrooms which were spread further by the bathroom exhaust fans - a total of 321 cases of Sars were detected in the flats.
Similarly, investigation in hospitals has found that faecal-derived aerosols in patients' toilets contained most of the detected Sars-CoV-2 virus. Positive isolates from toilet seats, handles and sinks were found even in the strictest isolation facilities in an infectious disease hospital in China in February.
Outbreaks in meatpacking factories have occurred in Germany, France, Spain, the US and the UK. Coronaviruses are more stable in cold, damp areas; the absence of daylight further facilitates viral survival. Add in crowded on-site accommodation, and you may have just devised the perfect breeding ground for Covid.
In Leicester, a rapid preliminary investigation into Covid-19 outbreak by Public Health England describes the number of new infections and their locations. The report found "no explanatory outbreaks in care homes, hospital settings, or industrial processes". It does not tell us about what is promoting the spread of Covid - such rapid analysis provides us with little insights.
Snow walked to Hampstead and back to question the late widow's butler; he did not rely on hearsay or indirect evidence. Similarly, to ensure he had the fullest dataset of deaths, he made door to door inquiries around Soho. We now need similar in-depth fieldwork. The resources to do this work already exist: universities have the infrastructure, the know-how and the skills to be rapidly deployed to investigate outbreaks - to go, door to door.
Presently the theories of how Co-V 2 is spread do not entirely fit the facts. Isolated outbreaks in meat processing plants around the world cannot be easily explained by droplet and respiratory spread. Contact and oro-faecal transmission are likely to complete the picture. It is even possible that droplets are aerosolised with contaminated toilet flushing one of the potential means of transmission.
Eradication of Co-V is not possible. Has Sars Co-V 1 been eradicated? We do not think so. Lack of evidence of its existence is probably due to us not looking for it in its inactive state. Viruses are all around us, but for the majority, the conditions are probably not right for their activation.
The data show the spread of the virus in workplaces is trending up while transmission in most other settings is in decline. We need John Snow epidemiological type fieldwork to formulate and test hypotheses and work out what's going on. A better understanding of the transmission of Covid-19 could cause a substantial shift in the current measures recommended to prevent infection.
