Thinking about a post-pandemic world

I've been trying to think carefully about what our world will be like once the current pandemic is over.  Most people are rightly focused on the current situation and on short term measures to limit the spread of the virus and the harm it causes, but we should also be thinking about, and planning for, what the world is likely to be like once populations reach some sort of equilibrium.  How many people will the virus be infecting or killing every year?  How much difference will a vaccine really make?  Will we still need to wear masks?

 What follows is my non-rigorous back-of-the-envelope analysis.  I made some big assumptions (spelled out) and did some simple arithmetic of a few simple scenarios.  There's no proper modeling here.

What kind of equilibrium we get will depend on how much immunity develops as a result of Covid-19 infection, how rapidly the immunity fades, and whether or not researchers can develop a vaccine that gives the same immunity.  So first we should consider some very broad-brush scenarios.

These outcomes range from very bad (no immunity, no vaccine) to quite good (lifelong immunity from a vaccine).  But how likely are they?  What kind of future should we plan for?

Based on what vaccine scientists have discovered and accomplished so far, I think that we will get a vaccine.  I also think that both ‘no immunity’ and ‘lifelong immunity’ are so unlikely that we shouldn’t waste time thinking about their consequences.

Now let’s do some planning for the most likely scenario.

Below I add more detail to the most likely outcome, that both infection and a vaccine confer moderate immunity.  I consider the effect of many people refusing (or being unable to pay for) vaccination, and of wearing masks and practicing some social distancing.

Let’s assume that the vaccine is a lot like our current flu vaccine, so it reduces your chance of being infected by about 50%, and your chance of having a severe infection or dying by 90%.  If you do get infected, it also reduces your contagiousness by about 50%.  Let’s also assume that you need to be re-vaccinated every year, both because the immunity fades over time and because in some years a virus mutation makes the current vaccine less effective.

What might the annual Covid-19 infection rates and death rates be in a typical year?  Let’s consider four scenarios that capture the most important possibilities:

• Scenario A: Many people refuse to be vaccinated (I assume 44% vaccine uptake, like the current flu vaccine), and social interactions return to normal.
• Scenario B: 44% of people refuse to be vaccinated, as in A. However most people practice some social distancing and wear masks when in crowds, which reduces the number of infections by about 50%.
• Scenario C: Most people are vaccinated annually, and social interactions return to normal. 
• Scenario D: Most people are vaccinated annually, as in C.  Most people also practice some social distancing and wear masks when in crowds, as in B., which reduces the number of infections by about 50%.

To estimate how much Covid-19 infection we might expect, I decided to use the CDC’s influenza data (https://www.cdc.gov/flu/about/burden/index.html).  The CDC estimates that influenza has caused 2,400-14,000 infections and 4-19 deaths per 100,000 people each year since 2010.  So I assumed that if Covid-19 vaccine works as well as the flu vaccine but with poor uptake and social interactions returned to normal, we would have about 7000 Covid-19 infections per 100,000 annually.  Initially I used the current Covid-19 death rate of 1% for unvaccinated people, and 0.1% for vaccinated people.

For comparison here are the extremes of the CDC’s estimates of the current influenza burden, drawn to the same scale:

So, if we get a vaccine that works only as well as the influenza vaccine, but most people get vaccinated, we could have an ultimate Covid-19 death burden lower than the current influenza death burden, even without social distancing and masks.

Some qualifications:

How soon will things stabilize?  Covid-19 is spreading rapidly in many parts of the world, and well controlled in others.  A vaccine like the one I consider will probably be available within a year or so.  It might then take a couple more years for vaccination and infection levels to settle into some kind of equilibrium.  But this will not be even across populations, and local outbreaks will continue to occur, since both vaccination rates and infection rates are likely to differ a lot between communities and between societies.

Covid-19 treatments may get better:  The analysis above assumes that the death rate from Covid-19 infection remains about 1%.  But Covid-19 treatments will probably continue to improve, reducing the death rate in both vaccinated and unvaccinated people.

Covid-19 infections often make people sicker than the flu, and for longer:  The analysis above doesn’t consider the disease burden of non-lethal Covid-19 infections, which appears to be much higher than that of influenza.  Although most people who get the flu feel lousy for a week or less, and return to full health within a few weeks, many Covid-19 infections cause more severe effects, with a wide range of debilitating symptoms that may linger for at least several months.  Because infections in vaccinated people are expected to be much less severe, employers (or insurers) might require proof of vaccination.

It's hard to predict the effects of changing contagiousness:  Above I've assumed that having been vaccinated reduces an infected person's ability to transmit the infection by 50%, and my calculations then assumed that this will reduce the overall number of infections at equilibrium by 50%.  But the dynamics of infection spread are complex, and the actual equilibrium reduction might be much stronger.