by Dr. Nell Thomas

Herd immunity is when the percent of human population that is immune to an infection – either from vaccination or from having previous infection – is high enough that each infected person transmits the disease to fewer than one individual.

It’s when the virus is unable to find susceptible people to infect and so it cannot replicate and spread. Without us spreading it, the virus dies out.

The percent of the population that needs to be immune is around 80 per cent for us to have herd immunity from COVID19 (for comparison, that number is around 65 per cent for influenza). As of April 5, about 15 per cent of Canadians have been immunized, and about 2.7 per cent of Canadians have had COVID19 (difficult to tally due to unreported cases, but best guesstimate). Not close to herd immunity, but clearly a lot more realistically achievable with our current daily vaccination rate of around 200,000.

Israel continues to be ahead of the curve and closing in on herd immunity. They accomplished this because the majority of their citizens are getting the shots. As a result, the pandemic is receding and they have seen a steady decline in new infections. About 56 per cent of their population has received their first vaccine. For the over-70 age group, they have more than 90 per cent vaccinated.

What does it look like here in Ontario for the vulnerable long-term care residents and for the health care workers? These two populations were targeted early in Phase 1 of our vaccination plan. On Feb. 28, 2021 there were 71 infected residents and 162 staff reported in Ontario’s COVID statistics. On April 4 there are six infected residents and 116 infected staff (many infected staff had opted out of vaccination).

Looking at the data from the latest analysis (the six-month update) from Pfizer there is significant cause to celebrate. There are zero hospitalizations among the vaccinated subjects compared to 32 hospitalized in the placebo group (unvaccinated) in a study of 46,307 people. And from South Africa where the variant B.1.351 is dominant, again zero cases of COVID among the vaccinated versus nine in the placebo group (study size 800 people). The vaccine is 100 per cent effective against severe disease as per the CDC definition. (By the way, no serious safety concerns were reported in the trial participants up to six months after the second dose, indicating it remains highly effective and safe. Looking forward to the nine and 12-month updates).

Let’s take a stab at the variants of concern. The Pfizer data from South Africa showed that six of those nine infections were with the B.1.351 South African strain, and so these data are consistent with previous studies showing that vaccinated individuals had good protection (a strong antibody response) against this variant.

Speaking of variants. The virus that causes COVID-19 (SARS-CoV-2) mutates about half as fast as the influenza virus. To survive, a virus must spread through the human population, and to do that it must reproduce itself. It does this by telling your cells to make copies of itself. Every time the virus is copied there is an opportunity for error in the virus’ genetic code. That code, by the way, is about 30,000 letters long. Lots of room for errors. Most of the errors make no difference, but sometimes an error results in a change that makes the virus infect people more easily. When that change starts to spread, it means a new variant emerges. When you consider how many people are actively spreading COVID-19 daily around the world, the handful of mutations that are significant are very few. Reassuringly, the vaccines currently being used are preventing severe disease, even from the new variants.


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