Archives for October 2020

By Rafaela Prifti- Today’s confirmation by the CDC that the pathogen is airborne brings together the science of overdispersion with the recognition of airborne aerosol transmission. Overdispersion is a key factor that should inform the approach to the pandemic and the preventive practices. Although much is still unknown about the super-spreading of SARS-CoV-2, nine months of epidemiological data shows that it is an overdispersed pathogen. It tends to spread in clusters, whereby one person tends to infect many or all that once, making them super-emitters of the virus. After months of extensive research by the global scientific community, many questions remain open about the substantial death toll suffered by a few cities compared to many others with similar density, household composition, weather, age distribution, and travel patterns in the spring of 2020. As the rest of Europe experiences a second wave, there are many explanations of variables such as weather, elderly populations, prior immunity, herd immunity—but there is a potentially overlooked way of understanding one factor: the measure of the dispersion of the pathogen. Overdispersion and super-spreading of this virus are found across the globe. A study found that in Hong Kong, which had extensive testing and contact tracing, about 19 percent of cases were responsible for 80 percent of transmission, while 69 percent of cases did not infect another person. In some cases of COVID-19 a single person did infect in excess of 80 percent of the people in the room in just a few hours, yet in other incidents COVID-19 is surprisingly much less contagious, even zero transmission. Multiple studies show that as few as 10 to 20 percent of infected people may be responsible for as much as 80 to 90 percent of transmission, and that many people barely transmit it.
SARS-CoV that caused the 2003 SARS outbreak was overdispersed in the same pattern. The majority of infected people did not transmit it, but a few super-spreading events caused most of the outbreaks. MERS appears overdispersed, but does not transmit well among humans.
The alternating between being super infectious and fairly noninfectious presents a huge challenge for health officials especially if we take into consideration that the pandemic playbook is based on the flu. Although a genuine threat, influenza does not have the same level of clustering behavior. Using a flu-pandemic playbook, won’t necessarily work well for an overdispersed pandemic.  To fight a super-spreading disease effectively, policymakers need to understand why super-spreading happens, its effects on testing regime and how to conduct effective contact-tracing methods.Experts divide the disease patterns into deterministic or stochastic: In the former, an outbreak’s distribution is more linear and predictable; in the latter, randomness plays a much larger role and predictions are hard, if not impossible. That means that the same inputs don’t always produce the same outputs.
Super-spreading clusters of COVID-19 almost overwhelmingly occur in poorly ventilated, indoor environments with over time congregations such as—weddings, churches, choirs, gyms, funerals, restaurants, loud talking, no masks. Studies show that the risk varies in every setting and activity. Infectious disease experts identify four key elements of super–spreader events: “prolonged contact, poor ventilation, highly infectious person, and crowding” as the key elements for a super-spreader event. Super-spreading can occur indoors beyond the six-feet guideline, because SARS-CoV-2, the pathogen causing COVID-19, can travel through the air particularly if ventilation is poor.
Given that some people infect others before they show symptoms, or when they have very mild or even no symptoms, it’s not always possible to know if we are highly infectious ourselves. There may be more factors but understanding the known elements of the pathogen’s behavior means that targeting clusters would be a very effective way in bringing down the transmission numbers. The health experts maintain that overdispersion should also inform our contact-tracing efforts. Right now, many states and nations engage in what is called forward or prospective contact tracing. Once an infected person is identified, we try to find out with whom they interacted afterward in order to warn, test, isolate, and quarantine these potential exposures. Backward tracing or retrospective contact tracking means identifying who was the first infected person. This approach is based in the nature of overdispersion since only a small percentage of people infect many at a time, whereas most others infect zero or maybe one.  Doing backward tracing to find the person who infected our patient, and then trace the forward contacts of the infecting person, rather than identify potential exposures, many of which will not happen anyway, because of the declining pattern of most transmissions    .
TEST AND TRACE METHODS
Public Health authorities consider that it doesn’t make sense to do forward tracing while not devoting enough resources to backward tracing and finding clusters, which cause so much damage. This point underlines the importance of rapid testing. The current dominant model of test and trace is not the best way when clusters are so important in spreading the disease, in terms of identifying people who are not infected and those who are infected. Although slow and expensive, PCR tests are highly accurate for both dimensions. However, PCR tests are slow, expensive and require a long uncomfortable swab, they are very accurate. Meanwhile, some rapid tests that are very accurate for ruling out individuals who are not infected with the disease but not as good at identifying infected individuals, are particularly valuable for cluster identification during an overdispersed pandemic. According to the specialists, this is helpful because some of these tests can be administered via saliva and be distributed outside medical facilities. Also increase the utility of wastewater testing which is effective for population-screening purposes, researchers say.
Because of the cluster behavior, identifying transmission events (someone infected someone else) is more important than identifying infected individuals using PCR testing. The most recent example of such a cluster was the Rose Garden event at the White House. Such cheap tests that could have been useful for the overdispersed pathogen. Yet these have been held up by regulatory agencies in the United States, due to the concern with their relative lack of accuracy in identifying positive cases compared with PCR tests.
ASSESSING DIFFERENT STRATEGIES
Many countries are now experiencing widespread rises in cases despite Europe’s relative level of success with containment rules this summer. The polarized debates about the pandemic bring up Sweden as an example to make a point about the efficacy of lockdowns. Studies show that Sweden like many other countries failed to protect elderly populations in nursing facilities, yet it has enforced stricter measures directed at super-spreading compared to other European countries – 50-person limit on indoor gatherings in March. Sweden encouraged social distancing and moved to online classes for higher-risk high-school and university students, while bringing the young children with a low transmission rate to schools—the opposite of the approach in the United States. Specialists say that “the most informative case studies may well be those who had terrible luck initially, like South Korea, and yet managed to bring about significant suppression.” In an overdispersion regime, the transmission level may be misleading. Just a few events can reignite massive numbers.
Japan did not impose a full lockdown, or interrupt mass transit. Like the US, Japan did not initially have the PCR capacity to do widespread testing. Recognizing the overdispersion characteristic of the pathogen, Japan strategy focused on cluster-busting including undertaking aggressive backward tracing to uncover clusters and counseling its population on ventilation. Japanese health experts say that restrictive rules are much more effective when they target the right key factor of the pandemic and work in combination with cheap testing and backward tracing to identify and limit the super-spreading events.