What Happens When We End The Lockdown?

As the number of COVID cases a day has passed its peak level and countries and states start to look at how to safely end their lockdowns and closures, what can we expect?  What would happen when we send people back to schools, work, stores and restaurants?

While it’s obvious to imagine that we would just see another explosion of COVID cases, the next phase will be different from March in several important ways:

1. We know how to identify it and are doing large scale testing across the country
2. Those with mild infections are unlikely to assume it's just a minor cold and go into work, school, or social settings where they could infect others. 
3. People will continue to demand safer environments from stores and public spaces that will reduce the chance of infection.
4. People will continue to keep habits of hand washing and many will continue to wear masks while COVID is present in the community

Even if we ended all the shelter at home policies, the actions and habits I've listed would still be in practice, so we wouldn't be returning to the same situation we had before.  While it may be difficult to see the dramatic effect these efforts have had on COVID because we're still catching up with it on testing,  you can see the effect its had on the flu when compared to last year:

But how can we estimate whether these remaining factors are enough to keep COVID in check?

The way scientists measure the growth rate of the pandemic is by researching samples of COVID patients and identifying how many people on average someone infected while they were contagious.  The number is referred to as R0 (and called R naught). When R0 is greater than one, the pool of active COVID cases grows (because the infected person replaces themselves and adds more beyond that).  When R0 is less than one, it shrinks (because the infected person doesn't fully replace themselves before they recover).  The acceleration of COVID seems to have stopped since we went into lockdown and did all the other things I described above, so the current growth number should be something less than 1. If we're worried about COVID cases exploding again if we lift the stay at home, what we're worried about is that COVID's growth factor would go back to being much higher than 1.  

To understand how COVID was growing before we made an effort to stop it, we can look to research.  There have been at least 137 studies done on R0 since the epidemic in different countries (but mostly China).    You can find them (as well as a nice summary of a lot of academic research into key parameters of COVID) here.  Averaging the R0 values from the 110 studies that were completed in the last two months gets us to 2.5  (most recent studies are very close to this number).  That is, in a starting situation where there is no immunity or counter measures, a person would typically infect 2.55 other people while they were contagious.  

To consider how R0 changed after we took action, there is the lockdown, social distancing, better hygiene, and other efforts I described.  Let's look at the impact of just the factor that people who develop COVID-like symptoms will usually self-quarantine until they can get tested and will avoid making contact with people until they are tested negative or recover from COVID.  When COVID first arrived in the US, it went undiagnosed and untracked.  Since 80% or more who get it experience mild symptoms, they likely made little effort to avoid infecting others and it spread quickly.  In mid-March when the state of emergency was declared and everything got cancelled, it would be nearly impossible to be coughing and running a fever and not suspect you have COVID -- even though at least 4 out of people tested get a negative result.  So, we would expect the amount of time a contagious person makes regular contact with their community to be much lower now.  

Other studies of COVID indicate that most people caught it from someone within 5 days of them showing symptoms.  If the average person now self-quarantines after two days of symptoms, that alone would reduce the growth rate of COVID by 60%, from 2.5 to 1.0 -- That's without any social distancing, lockdowns, or Purell.  When you include the other factors that would continue after lockdown is lifted, its very plausible that COVID cases would continue to shrink going forward.

It's important to point out that I'm NOT suggesting that we could safely drop all protective actions today.  I'm merely addressing the concern that ending the lock down would put us right back where we started.  Any model that projects the result of the end of the lockdown must factor all the other things that reduced the growth rate that will remain after lockdown if it is to give our leaders a realistic sense of their choices.