Top news, reports and insights for today:
- Daily headline summaries for Sunday:
- Evidence increasing that malaria drug hydroxychoroquine, backed by the White House, doesn’t improve outcomes and may cause harm (Washington Post)
- Texas among the three states (along with Wisconsin and North Carolina) that set a new record high case total on Saturday (1,801). Despite that, more businesses reopening there tomorrow (The Texas Tribune)
- The nation’s largest university system, California State University, announced that the fall term would be online across all 23 of its university campuses. Thousands of other colleges and universities may follow suit (Reuters)
- Dr. Tom Inglesby, director of the Johns Hopkins Center for Health Security says a vaccine is possible by the end of the year but that he wouldn’t “bank on it” (Politico)
- U.S. COVID-19 deaths doubled in 25 days: where we have been and where are we going?
Today’s analysis zeros in on COVID-19 doubling times for mortality across the country. The figure below tells the story of where we have been. The numbers are shown on the log scale, recalling that flattening indicates a transition from exponential growth (with short doubling times) to linear growth (with longer doubling times). The danger in using national numbers is that, as I have said in the past, there is no single U.S. outbreak; like politics, all epidemics are local. That having been said, there are a number of key points to be made about this graph:
- U.S. COVID-19 deaths doubled over the past 25 days. That still indicates widespread community transmission and fairly stable daily deaths.
- The doubling time has itself doubled 3 times since mid-March.
- In the log-scale, the cumulative epidemic curve for deaths is not yet close to flat. We can expect 1,000-2,500 daily deaths for some time to come.
- If the current trend continues, the next doubling time will be between 45 – 60 days. If true, the U.S. will have approximately 168,000 deaths by the middle of July.
- Does the SARS-CoV-2 virus spread through airborne transmission?
This week has seen several developments related to the vexing question of airborne transmission of SARS-CoV-2, the virus that causes COVID-19. For one-stop-shopping, check out this handy and timely overview article by Tanya Lewis in Scientific American that came out May 12.
Where were we 2 month ago? Initially, most experts believed that the primary mode of transmission is the deposition of respiratory droplets on surfaces that are then touched, bringing virus-laden particles to the airway, setting off a cycle of infection and viral replication in a new person. Based on how respiratory viruses work, and the sleuthing of epidemiologists based on initial values of R0, the belief was that airborne (aka aerosol) transmission was not really a thing. Early on, both the WHO and CDC down-played the risks of airborne transmission. Pathogens with sky-high R0 values (like measles at 14-18) are the best candidates for airborne transmission. Initially, the R0 for this new bug looked high-ish, but not in a way that pointed to a big role for airborne contagion.
Where are we now? Things have been moving fast, and while we still lack a definitive answer, several new clues have come onto the radar. First, two Chinese studies pointed to the possibility of airborne spread. The first, published in Nature on April 27 by Liu and colleagues found high concentration of viral RNA in air samples from patient bathrooms and in areas where medical personnel remove protective equipment. They only found evidence of viral RNA, not necessarily viable virus. A second study coming out in Emerging Infectious Diseases, documented a transmission chain linked to a family that ate at an air conditioned restaurant in Wuhan China in January, linked to 3 family clusters. Up to 9 secondary transmissions occurred that day, in people up to 3 tables apart. Most notably, the risk of infection was related to the direction of air conditioning currents, a fact that points strongly to airborne transmission. Then there is the mysterious clue that came from a cluster related to a choir practice in early March in Skagit County Washington. Sixty singers showed up for practice on March 6, all were given hand sanitizer at the door and all eschewed hugs and handshakes. No one wore masks; they sang for 21/2 hours. Three weeks later there were 45 cases and 2 deaths linked to the gathering. If you wanted to set up a ghastly experiment to test airborne transmission, this would have been it.
As these reports trickled in, engineers and physicists were starting to weigh in with basic science about the fluid dynamics associated with coughing, sneezing and talking. A study in the NEJM in mid-April found the SARS-CoV-2 virus remained viable in aerosols for up to 3 hours, leading to the conclusion that “aerosol and fomite transmission of SARS-CoV-2 is plausible, since the virus can remain viable and infectious in aerosols for hours and on surfaces up to days”. A few days ago, a paper in the PNAS by Stadnytskyi and colleagues used laser light to illuminate virus-bearing saliva droplets floating in the air following normal speech. They found that a minute of speaking could generate more than 1,000 infectious droplets with a hang-time of 8 minutes or more.
What this means: Taken together, the case in favor of some degree of airborne transmission is getting stronger. It’s also a nice illustration of the detective-like process of evaluating and combining evidence that characterizes epidemiological investigation in a novel outbreak. This virus is a killer, and it’s methods and motives remain mysterious. What we can say with some clarity, is that the old way of thinking – that there were two modes of transmission, droplets and airborne – has been replaced by an emerging consensus that sees these as outmoded, overly simplistic distinctions. Concretely, it means the initial 3 foot rule was ill-informed It gave way to the 6-foot rule, but now we know that airborne exhaust from sneezes and coughs can travel up to 30 feet. Two things are clear: 1) we can no longer assume that COVID-19 isn’t transmitted through airborne particles, 2) wearing masks in public is the right thing to do.