A short (I hope) thread about countermeasures
As the world's view of the #COVID19 spread turns from containable outbreak to pandemic, the clear next question is what countermeasures can be used. Categorizing these can be helpful.
1. Temporary vs. long-term/permanent. Most things we can do to slow the virus's spread -- isolation, quarantine, social distancing, canceling public gatherings, treating cases with antivirals if these are proven -- are temporary -- once let up, transmission can resurge.
On the other hand, vaccines if and when they become available could be a long-term solution. I hesitate to say permanent because we don't know what duration of immunity will be if vaccines are feasible at all. Can't know until we try. Immunity from natural CoV infection seems
to be relatively short-lived. But maybe vaccines can do better. So one category (long-term) is vaccines; the other is everything else.
2. Among the temporary countermeasures, some (case-based) depend on identifying cases and reducing their transmission (treatment, isolation, quarantine) and some don't (general social distancing, cancelling gatherings, encouraging cough hygiene/handwashing, etc.
Case-based interventions work best if the identifiably sick people are also the only or the main transmitters. They worked really well in SARS (2003), we think, b/c infectious people were all or almost all sick enough to be identifiable as cases.
For SARS-CoV-2 (#COVID19) it seems clear from individual well-documented cases that people can transmit before symptoms (or before they are distinctive enough to prompt suspicion of CoV infection) and that pre-symptomatic people shed virus (evidence from evacuation flights)
The key question -- for which we _desperately_ need good data, is whether this is common or rare. Some circumstantial evidence (eg the similar means of the incubation period and serial interval) suggests it could be common.
If transmission from people who are not identifiably sick is common, common sense as well as mathematical models by @ChristoPhraser pnas.org/content/101/16… and @coreypeak @Caroline_OF_B et al pnas.org/content/114/15… suggest that case-based interventions will be less effective.
@ChristoPhraser @coreypeak @Caroline_OF_B In sum, looks like case-based interventions for this infection will be partially effective but may not bring R-effective below 1 (R-effective or R_e is the average number of secondary cases from each primary case, which must be <1 to control the epidemic locally)
@ChristoPhraser @coreypeak @Caroline_OF_B 3. Among the non-case-based interventions, what is likely to work? This is perhaps the hardest to answer with a virus we don't yet understand well. Some have interpreted the decline in reported cases following "lockdowns" in China as evidence that these reduce transmission.
@ChristoPhraser @coreypeak @Caroline_OF_B That may be the case,but some of us are skeptical that the reported case numbers in China reflect the true numbers -- because ability to get to health care, get tested, and get a positive test may be limiting. IMO this is an open question.
@ChristoPhraser @coreypeak @Caroline_OF_B It seems likely these restrictions are reducing transmission somewhat, but how much is unclear. If presymptomatic transmission (more generally, transmission from individuals who aren't readily identified as cases) is common, then a relevant comparison is pandemic flu.
@ChristoPhraser @coreypeak @Caroline_OF_B Pandemic flu in 1918 was so severe that it was possible to estimate the impact of control measures by looking at the mortality rates weekly in different cities with different control measures. Two similar papers, one I was involved in, came out at the same time w similar findings
@ChristoPhraser @coreypeak @Caroline_OF_B pnas.org/content/104/18… and jamanetwork.com/journals/jama/… . Both found that cities with early interventions to close churches/theaters/schools/public gatherings had lower peak epidemics and smaller overall
@ChristoPhraser @coreypeak @Caroline_OF_B How relevant are these to the present? Hard to say. Key things to think about: a) schools. #COVID19 seems to cause confirmed cases more in adults than kids, though kids can be infected. Key to learn whether kids are important for transmission (maybe mild symptoms, but shed virus)
@ChristoPhraser @coreypeak @Caroline_OF_B If kids are important for transmission, school closures may help; if not, less so. B. flatter, longer epidemics. Models of disease transmission suggest that slowing transmission leads to flatter, longer epidemics cdc.gov/media/dpk/cdc-… 
@ChristoPhraser @coreypeak @Caroline_OF_B That is beneficial because 1) fewer individuals get infected in total; 2) the burden on the health system is spread out, with lower peak demand for scarce resources; 3) we will know more as time goes on about how to care for patients, so it is better to get it late than early;
@ChristoPhraser @coreypeak @Caroline_OF_B 4) if we can spread it out enough there may be new drugs, vaccines, or other countermeasures to aid in preventing and treating infection. Summary: Delay is good.
@ChristoPhraser @coreypeak @Caroline_OF_B So there will be arguments in favor of these kinds of countermeasures -- sometimes known as social distancing, nonpharmaceutical interventions (a broader class including case-based ones), or informally as quarantine or lockdown.
@ChristoPhraser @coreypeak @Caroline_OF_B It is key to remember that these countermeasures have costs. People gather for a reason -- for pleasure, for commerce, for companionship, etc. Preventing people from meeting will have psychological, economic, and other downsides which may be large.
@ChristoPhraser @coreypeak @Caroline_OF_B In the modern era, schools enable parents to work and are thus critical to the economy, as well as in some cases to the nutrition of kids (thru school meal programs). Lockdowns and other means of reducing contact are costly to those affected, in numerous ways.
@ChristoPhraser @coreypeak @Caroline_OF_B Depending on how they are done they may infringe civil liberties. But they may also save lives. A key agenda for the coming weeks is to learn as much as we can from populations with high case burdens about what works, what doesn't, to make better decisions about countermeasures.
@ChristoPhraser @coreypeak @Caroline_OF_B This thread has taken a global perspective so far, but from the US perspective, another issue is that people gather to vote, and we are voting in presidential primaries this spring. A key priority is to make sure that as concern about infection spreads, voting can proceed
@ChristoPhraser @coreypeak @Caroline_OF_B in a just and open way. This was meant to be a short thread. It wasn't short. Hope it was useful.
