As vaccination progresses in most countries, there is still the concern that although the vaccines protect against severe infection, they are less successful in preventing transmission, especially with the so-called “variants of concern”. Yet experts are even this week arguing as to what is the most important route for catching the virus. Public confidence and adherence to appropriate social distancing need them to be singing (but not too loudly!) from the same song sheet. (And that song might just involve cucumbers – see the end of this post!)
Aerosol or droplets as main route of infection?
- Readers I am sure will remember the discussion about whether Covid-19 is transmitted by large droplets or can be carried much further in the air as an aerosol
- Handwashing and social distancing to a metre are considered sufficiently protective against droplet transmission
- Indeed by 2 metres droplet transmission is unlikely
- Hence at the beginning of the pandemic the World Health Organisation (WHO) at their infamous press conference argued against the value of face masks
- WHO suggested that aerosol transmission was not an important route as Covid-19 was not airborne
- That opinion delayed widespread mask wearing and did not discourage social distancing beyond that short distance
- Much of the data for airborne transmission came from experiments in the laboratory trying to recreate what happens in life
- The argument was that the virus can transmit via the airborne route in the laboratory does not tell us how important that route of infection are in explaining who gets and who does not get infected
- But if airborne infection is the most likely route then too rapid a reduction in social distancing and mask wearing regulations could contribute to further surges in infection
World Health Organisation: are they wrong again?
- At the end of March this year a review of the available data, funded by WHO, suggested that airborne transmission could not be proven
- In that paper, which was widely publicised before peer review, Heneghan (from the Centre for Evidence Based Medicine at Oxford) and colleagues reviewed 90 research articles which had attempted to discover if Covid-19 could be airborne
- They applied their own stringent criteria to assess the quality of those research studies
- They rejected a firm conclusion because the quality of the research was not considered high enough
- As an epidemiologist I am (of course!) in favour of research being high quality, but such reports need to be careful in dismissing something as important for public health as this issue
- Indeed in the past 24 hours, the initial peer reviewers’ comments have just been posted and are not looking too good!
What is the evidence in favour of airborne transmission?
- Given the publicity for that WHO report there has been an rapid riposte.
- In an authoritative paper published in the Lancet a week ago, another colleague from Oxford, Trish Greenhalgh, gave 10 pieces of evidence which I believe would stand up in a court of law as proving more likely than not that airborne transmission was the key route of transmission
- These were her points:
- Superspreading events such as on cruise ships and choirs can only be explained by that route
- Transmission of infection between people isolating in their rooms, across corridors for example in those quarantined in hotels
- Perhaps as many as 1 in 3 of cases are transmitted from asymptomatic people who are not coughing or sneezing, and not passing out large droplets
- Transmission of SARS-CoV-2 is far higher indoors than outdoors and is substantially reduced by indoor ventilation
- Hospital and care home acquired infections have occurred despite very strict precautions against droplet spread and the limited effectiveness of much personal protective equipment (PPE) – which is designed to protect against droplet but not aerosol exposure
- Laboratory studies have shown that the Covid-19 virus can survive in the air and be infectious for up to 3 hours
- The Covid-19 virus has been found in air filters and building ducts in hospitals with COVID-19 patients
- Studies of infected caged animals which were connected to separately caged uninfected animals via an air duct and then caught the infection
- The data from contact tracing and interviews of infected people found no other convincing route of infection other than airborne
- Finally, although Heneghan and colleagues might argue against the quality of the research proving airborne transmission, the opposite has not been proven – that airborne transmission does not occur
The conclusion in that Lancet article is worth repeating here:
And the role of cucumbers:
- No this is not April 1st!
- Research published this week looked at how long the virus survived on various fresh fruit and veg food items that were unlikely to be cooked
- They inoculated test areas of the surface of these foods with the virus and measured the concentration of the virus on the surface at various subsequent time points
- Concentrations in terms of the number virus particles fell from the thousands over the next few hours
- As shown in the diagram, the virus had gone from apples and tomatoes within 16 hours but was still present on cucumbers at 72 hours
- Of course there are no data to suggest that cucumbers are in any way a likely source of infection, though interestingly the pH of the surface of cucumbers is more friendly to viral survival
- And remember you saw this important result on this blog first!
Take home message
I mention the cucumber study not because it is of major importance to the spread of infection. Rather to illustrate my main point that, especially in this pandemic, it is so important to judge all the research ‘in the round’
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