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COVID-19: Do The Vaccines Prevent Transmission?

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)

Before we get into today’s topic, I want to thank Canada’s longest running comedy show, This Hour Has 22 Minutes, for a funny bit at the end of this week’s episode that mocked “TV pandemic doctors”. They were kind enough to include photos of yours truly, and made me look like the hardboiled lieutenant in a 1970s cop thriller:

Today’s topic was inspired by this tweet by Cornell’s Brian Wasik, virologist:

Dr Wasik was responding to a statement by Fox News that, “We now know that the vaccines do not prevent infection and transmission, so any community-spread benefit is negligible.” Dr Wasik’s reply: “Vaccination reduces risk of infection. There is more than Omicron out there. Infection VE in the real world is clear. Look at Omicron-hit NYC through 12/25, 4X reduced risk.”

So let’s talk about whether these vaccines do reduce infection and transmission, and how we measure that.

Back in December of 2020, when the COVID mRNA vaccines were about to be authorized by Health Canada, I was being interviewed on CBC radio. The host asked, “What questions would you like to have answered about these vaccines?”

My response, “Do they just prevent symptoms, or do they also prevent transmission.”

The host was shocked and audibly gasped. “That seems like a big deal!” she responded, and clarified that she’d never considered that possibility. I was quick to point out that we have examples of vaccines that do not prevent asymptomatic infection, but which succeed in making the disease mostly go away anyway. Rotavirus comes to mind.

As the months have progressed, the question of the vaccines’ ability to prevent or slow transmission has become politicized, especially as new variants have emerged that challenge vaccine efficacy. What is certain is that the vaccines remain astonishingly good at preventing serious disease, hospitalization and death. There is no credible argument against that, as per these data from Alberta, which show that the hospitalization rate among thrice-vaccinated 80+ year olds is lower than the hospitalization rate among unvaccinated 12-29-year-olds:

And these data from Switzerland, showing that the unvaccinated are far more likely to die. This is doubly astonishing since the unvaccinated are more likely to be younger and healthier than the vaccinated:

When looking at risk of infection, though, the story is not as clear. As noted in my previous post, “Let’s Talk Again of the Base Rate Fallacy“, since Christmas Ontario is showing a paradoxical increase in infection risk among the vaccinated, which I attribute to a combination of testing bias and exposure bias.

So Omicron might have confused the issue somewhat. Let’s try to disentangle it a bit more. (more…)

COVID-19: Let’s Talk Again of the Base Rate Fallacy

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)

Since the start of the pandemic, the general public has received a rapid education in some of the terms with which only Epidemiologists used to be familiar. Ideas and words like “herd immunity” and “reproduction number” are now casually bandied about people who had never heard the word “epidemiology” before 2020. (more…)

2022

2022

This website is now 29 years old. It’s older than most of my students. It’s older than the Lisbon Treaty and almost as old as the Maastricht Treaty. It’s definitely older than most pop stars, beauty queens, and social media influencers. But it’s never quite as old as me. (more…)

COVID-19: Will It Mutate to Nothingness?

https://daily.jstor.org/viral-mutation-for-the-perplexed/

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)

My sister recently asked me if viruses like the one that causes COVID-19 always mutate, or evolve, toward less lethality. This is a common question these days since new COVID variants are now appearing regularly, and since COVID misinformation merchant Dr Kelly Victory famously claimed that as viruses mutate, they “become more contagious/transmissible and less lethal.”

This is decidedly not true, as I will try to explain below. But before I do so, here’s my caveat: I am not a virologist or an evolutionary biologist. I’m a lowly global health epidemiologist who observes and studies a variety of diseases and health states on the world stage. In this post, I’m interpreting and translating the work of others. (more…)


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