TORONTO – Due to the increased transmissibility of the Delta B.1.617.2 variant, now dominant in Ontario and much of the world, the percentage of the population that needs to be vaccinated to reach a point where the long-term spread of the virus slows down, now exceeds the number of people medically entitled to vaccination. “The critical threshold for vaccination should be at least 90% of the Ontario population and more than 100% of the vaccine-eligible population,” says Public Health Ontario. 

The original target of the vaccination rate required to move to the next reopening phase and fully restart Ontario was 75% of people over 12 years of age fully vaccinated, 80% with at least one dose and no region with less than 70% of full vaccinations.

Public Health Ontario says the goal of vaccinations to be achieved was developed against the characteristics of the Alpha B.1.1.7 coronavirus variant, which research now suggests is much less deadly than the Delta.

“Often people get angry, they say ‘the cards on the table keep changing’ and the answer is yes, of course ‘the cards’ are changing because the virus has mutated – said the infectious disease specialist of the UHN, Dr Isaac Bogoch in an interview with CTV News Toronto – this is a much more transmissible variant, which means that a larger percentage of the population must be vaccinated to keep the virus under control”.

A study published this month in the Journal of Travel Medicine found that the Delta variant is almost twice as transmissible as all variants that preceded it, with an R0 of 5.

But, according to Ontario Chief Medical Officer of Health Kieran Moore, water under the bridge has yet to pass. Moore said he does not expect the spread of Covid-19 to start slowing down any time soon, but not until 90% of the population is fully vaccinated, which is expected to occur in the spring of 2022.

In its latest report, Public Health Ontario states that these earlier goals are now not sufficiently valid, and the continued arrival of foreign travelers to the province “challenges the applicability of herd immunity concepts to the current context.”

Herd immunity is generally defined as the point at which a sufficient number of members of the population have developed resistance to infection of the pathogen by means of antibodies generated through a previous infection or vaccination.

But it will take months to get vaccinated children under the age of 12. “Kids under the age of 12 will not be vaccinated in time before returning to in-person learning in September, and are therefore a population where Delta will circulate as community-wide public health measures are lifted,” the doctor noted.

But Bogoch also said that authorities should not rush trials examining the effects of vaccines on children in order to implement them in the early part of the school year.

On the same page of the Canadian health officials is also the Italian epidemiologist Stefania Salmaso: “Herd immunity is not an achievable goal in this context. We were deluded at the beginning – she observed – but herd immunity exists and is established when in the population, in the flock precisely, there are so many vaccinated that these act as a shield to those very few who are not vaccinated and who are dispersed among others. But that is not the case. Meanwhile we have the unvaccinated who are all in the very young age group and therefore cannot be vaccinated. And then we have seen that vaccines, although very effective, do not completely protect against infection, they do not always act as a total shield for the transmission of the infection “.

“So – she concluded – the message must be clear: those who are not vaccinated are not protected from others, those who are vaccinated are protected above all from the most severe events related to the infection. The idea that reaching 80% of vaccination coverage means saying that we have achieved herd immunity, when we have all the children unvaccinated, is a discourse that scientifically is not solid “.