Superspreading Event

Just how many Americans have been exposed to COVID-19 is a difficult number to know with absolute certainty but each day we get closer and closer as serological (antibodies to the virus present in the blood) studies begin to roll in. Many Americans drastically overestimate this number, perhaps as some form of wishful thinking. Current best estimates from ongoing serological studies from the CDC and others puts the positivity rate somewhere around 10% or less nationwide with some regions higher and some lower based on regional disease prevalence to date. Current deaths due to COVID-19 stand at just over 214,000. To obtain herd immunity for COVID-19 we need to reach, at a minimum, 70% of Americans infected. With 214,000 deaths after the first 10% of Americans infected we could expect another one million deaths or more before herd immunity would have any significant protective effect. Even if our ability to treat patients improves enough over time to cut the death rate in half we’d see an additional half a million deaths from a do-nothing approach to the pandemic.
COVID-19 denialists hold that deaths due to COVID-19 are over-counted. A few weeks ago they gleefully pointed to a CDC report which revealed that 94% of reported deaths were in people with other health issues. To them, this apparently indicated that only 6% of people had actually died from COVID-19. I’m not going to address the fallacy of that thinking here other than to say that if I have diabetes and while crossing the road I get struck by a bus and die, my cause of death is blunt force trauma–pure and simple. We can’t say that diabetes killed me in this case even if my diabetes made my treatment more difficult and played some role in my demise. Diabetes may very well be listed on my death report; I still died from blunt force trauma. The clearest evidence that denialists are wrong here comes from an examination of excess deaths. Epidemiologist can easily examine the previous five or ten years of deaths to establish a predicted number of deaths for the current year. In 2020, as of mid-September, there were 278,000 excess deaths compared with previous years. At that time (September 19, 2020) there were 199,000 deaths attributed to COVID-19. There are two important points here that invalidate the unscientific thinking of COVID-19 denialists in several ways. First, an accounting of excess deaths isn’t an examination of cause of death. It’s simply this: let’s count deaths from the last few years and then let’s count deaths from this year. Guess what, the number this year is bigger…by quite a bit. Where did all those excess deaths come from? It didn’t come from the soul-crushing inability to go to the gym or a favorite nail salon; it came from people dying after being infected with a novel coronavirus. The second important point form this data is that the evidence points to the fact that we’re undercounting COVID-19 deaths. As of September, there have been 278,000 excess deaths but only 199,000 (currently 214,000) deaths reported as due to COVID-19. That means there are nearly 80,000 extra deaths this year that very likely the majority of those were uncounted COVID-19 deaths.

CDC Weekly Excess Deaths showing significant increase above predicted beginning early 2020 and the observed deaths exceeding those attributed to COVID-19. https://www.cdc.gov/nchs/nvss/vsrr/covid19/excess_deaths.htm#dashboard

A report from the CDC shows that Arizona’s institution of mandatory masks significantly reduced transmission of COVID-19 within 2 weeks. A combination of mask mandates and other mitigation strategies (prohibiting large gatherings, temporary closure of bars, gyms and theaters and a reduction in restaurant capacity led to a 75% decline in COVID-19 transmission statewide.

The number of COVID-19 cases stabilized and began to decrease approximately 2 weeks after local officials began mandating mask wearing (throughout several counties and cities) and enhanced sanitation practices
https://www.cdc.gov/mmwr/volumes/69/wr/mm6940e3.htm?s_cid=mm6940e3_w

A massive study out of India looking at COVID-19 transmission dynamics is well worth reading. I’m going to focus on two important points from the study in part because of their timeliness. First, the study showed that not all spreaders are equal. Approximately 70% of infected individuals did not demonstrate spread to other contacts. Instead, 8% of positive index cases were responsible for nearly two thirds of contact cases. This supports the growing evidence that super-spreading events such as the White House Rose Garden SCOTUS event play an extremely import role in disease spread and underscores the importance of social distancing, mask wear and restricting large groups. The second point I want to highlight from the Indian study shows that children spread the virus at similar rates as seen in adults. The study was unable to examine spread from children to adults but it did show that infected children spread the disease to 18% of their peers. This of course has significant implications for the reopening of schools across the country. As I’ve stated before, to safely reopen schools disease prevalence in the overall community must be very low and testing must be widespread with quick results to allow for rapid isolation of positives and contacts and comprehensive contact tracing. This kind of low community rate comes from grown-ups strictly adhering to non-pharmaceutical interventions such as but not limited to full adherence to mask-wearing and the avoidance of congregating in group settings.

CDC graphic shows spread through 87% of choir practice participants from one symptomatic individual/cdc.gov MMWR

The Morbidity and Mortality Weekly Report (MMWR) by Lea Hamner et al. released today describes the impressive spread of SARS-CoV-2 from one symptomatic individual to 53 (33 confirmed and 20 probable) out of 61 members of a choir in Washington State. Of those infected, two eventually died.

The report startlingly illustrates some important phenomena related to rapid viral spread. First it’s important to understand that there are likely individuals who inherently spread more virus than the average person or there are at least circumstances that facilitate greater spread. A study by Asadi S. et al. in 2019 showed that the louder the voice, the greater the aerosol emissions. This is likely also true whenever forceful exhalation occurs. With activities like singing or vigorous exercise there comes the possibility that some individuals may be considered super-emitters who spread a much greater amount of virus-containing aerosols than typical individuals in normal situations. In this case the symptomatic choir member’s singing may have made them a super-emitter. On top of that, prolonged exposure in an enclosed space with little to no social distancing was an additional setup for aggressive viral spread through a large group; this is know as a superspreading event (SSE). SSEs are important public health concerns and have been documented in many pandemics. In a COVID-19 SSE, an infected individual can spread the virus to far more than the usual 2-3 people a typical person with SARS-CoV-2 does. In the Washington SSE for instance, the symptomatic choir member likely passed the virus on to dozens of people. SSEs are more likely to occur during gatherings at churches, concerts, sporting events, birthdays, funerals and at gyms or fitness centers. In each case we see the potential for a dangerous combination of people with loud voices or heavy breathing (super-emitters) coming together with large groups of people in enclosed spaces with less than optimal social distancing to create SSEs.

This report highlights the logistical challenges with large-scale reopening of society and businesses in the era of COVID-19, particularly for the highest risk sites. When venues such as those listed above do open, great care must be taken by both organizers of the venue and by visitors to the venue to reduce the risk of SSEs. Even with substantial attention and effort, it remains to be seen whether high-risk venues will be able to convince skittish visitors or members that it is safe to return as long as the virus is present in the community or until an effective vaccine is available.