TORONTO -- Achieving herd immunity against COVID-19 will not be possible without an approved vaccine for children under 12, Public Health Ontario says in a new report about what the pandemic will look like this fall and winter.

Due to the increased transmissibility and partial vaccine escape ability of the Delta B.1.617.2 variant now dominant in Ontario and much of the world, the report says the percentage of the population requiring vaccination to hit a point where virus transmission enters long-term decline now exceeds the number of people actually medically eligible for vaccination at this time.

“The critical threshold for vaccination is now estimated to be at least 90 per cent of the Ontario population, and over 100 per cent of the vaccine-eligible population,” Public Health Ontario says.

The original target vaccination rate called for to fully exit Ontario’s reopening plan was 75 per cent of people over 12 fully vaccinated, 80 per cent with at least one dose and no region with less than 70 per cent fully vaccinated.

Public Health Ontario says this vaccination target was developed against the characteristics of the Alpha B.1.1.7 coronavirus variant, which Ontario research now suggests was far less deadly than Delta.

“You often hear in the general public people get upset, they say ‘the goal posts keep changing,’ and the answer to that is yes, of course the goal posts are changing because the virus has changed,” UHN infectious diseases specialist Dr. Isaac Bogoch told CTV News Toronto.

“This is a much more transmissible variant which means you have to vaccinate a greater proportion of your population in order to keep the virus under control.”

Part of how the herd immunity threshold is calculated involves finding the virus' reproductive number or R0.

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

Chief Medical Officer of Health Dr. Kieran Moore said Tuesday that he does not expect COVID-19 transmission to enter long-term decline until 90 per cent of the population is fully vaccinated, which he expects to occur sometime in spring 2022.

In its latest report, Public Health Ontario says those previous targets are now not strong enough and the continued arrival of foreign travellers to the province “challenges the applicability of herd immunity concepts to the current context.”

Herd immunity is broadly defined as the point at which enough members of the population have resistance to infection by a pathogen either by antibodies generated through prior infection or vaccination.

Moving to vaccinate those still ineligible, children under the age of 12, is still at least months away.

“Individuals under 12 years of age will notbe vaccinated when they are expected to return to in-person learning in September, and are therefore a population where Delta will circulate as more community-level public health measures are lifted.”

But Bogoch said authorities should not rush trials examining the effects of vaccines on younger people so that they could be rolled out in the early part of the school year.

“We should not rush any science, we have to let the clinical trials be completed so that we can ensure when populations under 12 are vaccinated we have an excellent safety profile and efficacy of vaccines in that cohort.”

Public Health Ontario suggests developing different thresholds for COVID-19 cases warranting further action in daycares and elementary schools, where children will be unvaccinated, versus secondary schools where children are eligible for vaccination.

They also suggest encouraging parents of elementary and daycare-age kids to get vaccinated.

“It may be useful to survey the vaccination status of eligible household members living with the children attending childcare and elementary schools to identify opportunities to support vaccine confidence and uptake and thereby reduce the risk of chains of transmission extending to households or childcare/school settings.”