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COVID-19: Even More Q&A – deonandia

COVID-19: Even More Q&A

by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

Today’s post is a bit of a hodgepodge of topics people have been asking me to talk about. Let’s begin with a question from a concerned citizen, who asked:

A) “How Will The Pandemic End?”


“Hi Dr. Deonandan. I know you are very busy but I was wondering if you could explain how this pandemic will end. I know that pandemics always end. However I don’t understand how this can end with so many people not getting vaccinated. I know more countries across the world have to be vaccinated but I don’t see how Ontario or Canada will ever get enough people vaccinated. There are just so many people that don’t want to get the vaccine and won’t get it. Is there some other way it can end?”

Here’s a modified version the response I sent:


All pandemics end in more or less the same way: when there aren’t enough susceptible people who can easily become infected.

A susceptible person is one with no immunity. That means the unvaccinated or those who have never contracted and recovered from COVID.

So imagine if a vaccine were never found; then the disease would tear through the population, killing about 1% of everyone it infected, until it ran out of hosts to infect. This sounds like an appealing strategy to some, but keep in mind that 1% of a very large number is an astonishing number of deaths. And the 99% who survive would suffer some long term disability, but would retain some immunity for many months.

If you look at the Spanish flu, it tore through the world until it mostly ran out of hosts. But came back a year later, having mutated slightly, but had a harder time finding hosts because many people had acquired some level of immunity. The Spanish Flu was likely stopped by a combination of mutation and population immunity through recovery.

The descendants of the Spanish flu rear their ugly heads every few decades and cause a lot of problems but don’t really get much of a foothold because enough people retain some immunity memory.

With COVID, eventually enough people will have at least some immunity that the disease will struggle to find sufficient hold to cause a world-stopping crisis again. But it’s not going away completely, the same way that the Spanish flu never went away completely. It just won’t be the killer that it is now.

We are lucky that we have vaccines. We can push artificial immunity into more people faster and don’t have to rely on the ugly brute force of natural infection, which necessarily comes with death and suffering.

Epidemics are modelled using compartments we label as “S”, “E”, “I” and “R”. S stands for “susceptible”, which is everyone at the start. E stands for “exposed”, which is what we are when we don’t apply mitigation tools. I stands for “infected”, which is what many of us become once we are exposed. And R stands for “removed”, which is what happens to us when either we die or we become infected and recover, thus acquiring immunity.

Vaccination moves people directly from S to R without having to go through E and I. But regardless, when enough people move from S to R, and S becomes small, then the pandemic is effectively over.

B) “The Vaccines Don’t Prevent Transmission So What’s The Point of Them?”
I tweeted this image taken from an article by Sarah Caddy, though I added the blue bubble:

The prevention of any infection at all is the holy grail of vaccine science. We call this “sterilizing immunity”, when a vaccine creates such robust immunity in a person that the targeted virus can’t even get a foothold in the body. But sterilizing immunity is actually quite rare. It’s one of the reasons I have always been extra bullish on the Moderna vaccine, as it’s the only COVID vaccine to have shown signs of sterilizing immunity in its animal models. (And part of why I’m so frustrated by people who prefer Pfizer over Moderna….. people, if you only knew the data!)

When a vaccinated person becomes noticeably infected, we call this a “breakthrough” infection. Though some have suggested we call it a “spillover” instead, since “breakthrough” implies that a wall of immunity was penetrated. In fact, that wall remains standing strong; it’s just that there is so much virus circulating, that some of it “spills over” to affect a few people, most typically those who failed to mount a sufficient immune response.

As you can see from the image above, vaccines see first to prevent death and serious disease. As a bonus, some prevent mild symptoms and fewer still will prevent asymptomatic infection. The COVID-19 vaccines are astonishing in their ability to prevent death and hospitalization and serious infection. Their effect is indicated in the blue bubble I’ve drawn in the image. They’ve essentially reduced COVID-19 to a bad cold. And is that not good enough? The risk of a bad cold should not fill us with fear or shut down society.

In the USA, 99.5% of COVID deaths are among the unvaccinated. Once infected, the unvaccinated are 29 times more likely to be hospitalized than the unvaccinated. And in terms of actually becoming infected in the first place, the unvaccinated are at least twice as likely as the vaccinated to become infected.

Anti-vaxxers and some panic-friendly media have been quick to amplify the news that with respect to the delta variant, infected vaccinated people have similar nasal viral loads to infected unvaccinated people. This is in contrast to experiences with pre-delta variants which consistently showed that in breakthrough infections, viral load was considerably smaller.

What this means is that those infected vaccinated persons are just as likely as unvaccinated persons to pass it on to someone else while they’re infected. That’s not great. But there are two very important caveats here:

  1. The vaccinated person first has to become infected, which I’ve pointed out above is much less likely to happen; and
  2. Once infected, vaccinated people carry that viral load for a shorter period of time, so are able to infect much fewer people during their infectious period.

Bottom line: at the population level, vaccination totally reduces overall transmission. It ain’t even close.


C) “But Vaccinated People Comprise the Majority of Cases and Hospitalizations!”

First, this isn’t true. Second, it will become true very soon. Third, when it becomes true, this will be a good thing.

This is an example of what Epidemiologists call the “base rate fallacy”. In short, as more people become vaccinated, we fully expect two things: that cases will eventually come down, but that increasingly those cases that do occur will be mostly among the vaccinated…. simply because more people are vaccinated.

The analogy I’ve been using is to seatbelts. When 100% of drivers wear seatbelts, the number of car crash fatalities drops significantly, but not to zero! Those fatalities that still occur will all be among people who wore seatbelts. It doesn’t mean that seatbelts don’t work! What matters is whether the proportion of fatalities among seatbelt wearers is less than the proportion of fatalities among non-seatbelt wearers.

Same logic applies to vaccination.

I heard someone report a while back that 67% of COVID cases in Iceland were among the vaccinated. Cue anti-vax cries of glee. Keep in mind that Iceland has about 74% of their population fully vaccinated.

More well known is the example of 50% of Israel’s cases being among the vaccinated, and Israel has a vaccination rate of >60%, and >85% of their adults. Cue this image stolen from “yourlocalepidemiologist“:

It shows the Israeli adult situation in which 85% of the population is vaccinated, and 4 people in this theoretical population of 100 people have COVID.  If you only look at the infected population (or the hospitalized population) then you’d see that 2 out of 4 cases –50%– are vaccinated, and would erroneously conclude the the risk of infection among vaccinated is equivalent to the risk among unvaccinated. False.

Risk is calculated as the number of cases divided by base population. Among the vaccinated in this example, the risk of infection is 2/85 or 2.4%.  And the risk among the unvaccinated is 2/15 or 13.3%.  The risk among the unvaccinated is therefore more than fives times greater than the risk among the vaccinated. But you’d never see that by just looking at the cases or hospitalizations.

There will come a time when we have 100% vaccination, but not zero cases. (Because the world is not perfect). When that happens, 100% of cases will be among the vaccinated. That does not mean that vaccines don’t work. Rather, it means that they work brilliantly to diminish the epidemic, though not to eradicate infection entirely.

D) “You Got Your Shot. So Why Do You Care If I Don’t Have Mine?”

I enjoy the philosophy of John Stuart Mill. But the dude’s distilled mores really are an impediment to public health. It’s true that your rights end where my nose begins. But what’s unclear to many is that very often my nose and your nose are intimately intertwined.

Did I mix too many metaphors?

I made a list of reasons why we vaccinated people care that others are not vaccinated:

1) Breakthrough infections of vaccinated people are not the norm, but happen when transmission rates are high. Being around unvaccinated people during an epidemic raises the probability of exposure, thus raising the probability of a breakthrough infection.

2) Despite what your Facebook friend has told you, vaccination does reduce transmission to s high degree. (See above). So to avoid the scenario in (1) we want more vaccinated people in the community to drive overall transmission rates down. That’s also how we keep infection out of schools and workplaces.

3) The ineligible (kids under 12) and those who did not mount an immune response cannot be protected by vaccine, so must be protected by herd immunity. For the delta variant, that means likely getting >85% of people jabbed. So it matters to us that you’re not vaccinated because your lack of immunity places our children at risk.

4) New variants emerge when many people become infected. Since vaccinated people are less likely to be infected, it’s the unvaccinated who are the likely variant farms. And if they produce a vaccine-escape variant, then the vaccinated people are at risk, too, and this nightmare goes on longer.

5) Right now in Canada, a large proportion of people remains unvaccinated, which means a large number of people can get sick and still become hospitalized. This is the susceptible population. (See above.) That’s a strain on our health care system that threatens quality of overall care to everyone, vaccinated and unvaccinated alike.

6) We love you and we want you to be healthy and not to die.

Until next time!