One of the many mysteries of the Covid-19 pandemic is how the disease actually spreads. We were told to wash hands and about the dangers of droplets in one to one contacts. But newer evidence continues to point in another direction.
There are more and more reports about cases where the infections seems to have spread by aerosol, droplets smaller than 5μm in diameter, instead of by bigger droplets or fomites like surfaces and objects. Lambert Strether has collected reports of cluster cases in restaurants, buses, ships and a callcenter where aerosol transmission was the most likely cause:
There's mounting evidence that airborne transmission indoors is a key - perhaps the main - pathway to SARS-COV-2 transmission. In this post I want to look at why that's so, give examples, and suggest a simple heuristic to stay safe.
But a recent report about a South African study of a hospital cluster where 119 patients and staff caught the disease claims that most infections must have come through fomites transmission:
[O]n the whole, patients infected few other patients directly. Instead staff members spread the disease from patient to patient and from department to department-perhaps sometimes without becoming infected themselves. "We think in the main it's likely to have been from [staff] hands and shared patient care items like thermometers, blood pressure cuffs, and stethoscopes," says Richard Lessells, an infectious disease specialist at the KwaZulu-Natal Research Innovation and Sequencing Platform and one of the study leaders. He and the other authors found no evidence that aerosol transmission contributed to the outbreak.
A look into the very detailed and otherwise excellent original study shows why the researchers found no evidence for aerosol transmission. They never looked for any. The words 'air conditioning', 'HVAC' or similar do not appear in the 37 pages. That is because right at the start of the study the researchers excluded the possibility that aerosols may play a role:
SARS‐CoV‐2 is thought, on the basis of current evidence, to be transmitted between people through respiratory droplets and contact.
Whilst aerosol transmission may be possible in specific circumstances, particularly in the healthcare setting with aerosol‐generating procedures (i.e.endotracheal intubation, open suctioning, and manual ventilation before intubation), there is currently no evidence that aerosol transmission is an important mode of transmission more generally.
That paragraph is footnoted with a link to a WHO recommendation from March. Since then much has been learned about cluster cases in which aerosols were the most likely transmitter of the disease.
Aerosols are droplets smaller than 5 micrometers. At that small size they do not fall to the ground but float in the airstream. Unlike droplets they are not a problem outside of closed rooms as the normal air movement will start to disperse them immediately.
A study in Hubei tracked down 318 cluster creating incidents in which at least 3 persons were involved. It found that only one happened in open air. A Japanese study says that the risk of infection indoors are 19 times higher than outdoors.
An early study has found that the secondary attack rate in households defined as "the probability that an infection occurs among susceptible people within a specific group (ie, household or close contacts)" is quite low at some 35%. Other studies have come to even lower values of some 25%. There were a number of reports of families where only one or two persons were infected while other members of the household did not catch the disease.
But the overall reproduction rate R0 of Covid-19 is estimated to be somewhere between 2 and 3. That means that without isolation measures each newly infected person will on average infect 2 to 3 other persons. How does that fit with the relatively low secondary infections in households?
Science has published a must read piece that explains this conundrum:
Other infectious diseases also spread in clusters. But COVID-19, like two of its cousins, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), seems especially prone to attacking groups of tightly connected people while sparing others. It's an encouraging finding, scientists say, because it suggests that restricting gatherings where superspreading is likely to occur will have a major impact on transmission and that other restrictions-on outdoor activity, for example-might be eased.
Most of the discussion around the spread of SARS-CoV-2 has concentrated on the average number of new infections caused by each patient. Without social distancing, this reproduction number (R) is about three. But in real life, some people infect many others and others don't spread the disease at all. In fact, the latter is the norm, Lloyd-Smith says: "The consistent pattern is that the most common number is zero. Most people do not transmit."
That's why in addition to R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k is, the more transmission comes from a small number of people. In a seminal 2005 Nature paper, Lloyd-Smith and co-authors estimated that SARS-in which superspreading played a major role-had a k of 0.16. The estimated k for MERS, which emerged in 2012, is about 0.25. In the flu pandemic of 1918, in contrast, the value was about one, indicating that clusters played less of a role.
Current estimates of the dispersion factor k for SARS-CoV-2 vary between 0.1 and 0.5. That means that cluster infections from relatively few superspreading events drive the epidemic more than single transmissions from one person to another person.
This explains the success of the Japanese strategy which brought the epidemic in that country down without ordering strict lockdown measures:
As of Thursday, Japan had confirmed more than 16,000 infections and about 900 deaths from the virus, by far the lowest figures among the Group of Seven major economies.
Japan has urged people to avoid environments with what it calls the "Three Cs", meaning close contact in closed-off, crowded spaces, where experts say most infections have occurred.
Without knowing if the measures would work Japan picked the right strategy. Only those events and places where superspreading is most likely to occur where shunned. Additionally people in Japan actually wear their masks and are generally health conscious and disciplined.
Unfortunately it is unlikely that 'western' nations will develop such discipline.
Yves Smith has written about her recent personal experience in a hospital in Alabama where even the staff were not wearing masks and were also otherwise quite careless. This at a time where numbers in Alabama are surging.
A number of 'western' publications have claimed that democracies are more effective in fighting an epidemic.
The U.S. has now more than 100,000 deaths from Covid-19 and Britain has the highest rate of death in the world:
The UK has registered 59,537 more deaths than usual since the week ending March 20, indicating that the virus has directly or indirectly killed 891 people per million.
At this stage of the pandemic, that is a higher rate of death than in any country for which high-quality data exist. The absolute number of excess deaths in the UK is also the highest in Europe, and second only to the US in global terms, according to data collected by the Financial Times.
China, Vietnam and other countries that the U.S. likes to call dictatorships have handled the crisis much better than the so called leading democracies. In total all Asian nations seem to have had a better grip on the epidemic than many other countries. The most likely reason for this is that those cultures care far more for the collective good than for the benefit of individuals.
The "western" cultures allow for more selfishness of the individual. But over the longer timeframe cultures that emphasizes personal liberty and ignore the common good are likely to see their empire fail.
Probably the biggest lesson we will learn from this pandemic is that we must work to change that selfish mentality.
Posted by b on May 28, 2020