While studies have not yet collected four months of data on vaccine effectiveness after the first dose, the first two months of real world effectiveness are showing sustained high levels of protection. ...
Short term sustained protection is consistent with immunological principles and vaccine science where it is not expected to see rapid waning of a highly effective vaccine in adults over a relatively short period of time. Extending the interval between doses was shown to be a good strategy through modelling, even in scenarios considering a six month interval and in theoretical scenarios where waning protection was considered.
NACI recommends that in the context of limited COVID-19 vaccine supply, jurisdictions should maximize the number of individuals benefiting from the first dose of vaccine by extending the interval for the second dose of vaccine to four months.
Their argument seems to be that we have 2 months of evidence that hospitalizations & negative effects are strongly reduced even by single doses, and we can reduce world-wide virus numbers by getting more 2x single dose resistances rather than single person double doses.
In this view, the risk of resistant strains would actually be less with 2 people at ~85% resistance, than one person with 95%. This is because if everyone had 85% resistance the virus reproduction rate would be <1, and it would completely die out, so wouldn't have an opportunity to evolve stronger strains - which it currently does have due to being prevalent all over the world.
When vaccines are in limited supply, expanding the number of people who receive some vaccine can reduce disease and mortality compared to concentrating vaccines in a subset of the population. A corollary of such dose-sparing strategies is that vaccinated individuals may have less protective immunity. Concerns have been raised that expanding the fraction of the population with partial immunity to SARS-CoV-2 could increase selection for vaccine escape variants, ultimately undermining vaccine effectiveness. We argue that although this is possible, preliminary evidence instead suggests such strategies should slow the rate of vaccine or immune escape. As long as vaccination provides some protection against escape variants, the corresponding reduction in prevalence and incidence should reduce the rate at which new variants are generated and the speed of adaptation. Because there is little evidence for efficient immune selection of SARS-CoV-2 during typical infections, these population-level effects are likely to dominate vaccine-induced evolution.i.e. if you think 1x0%, 1x95% is better than 2x85%, do you not also think that 3x0%, 1x99% is better?
It sounds like Canada’s approach is aware of this as it is “extending the second dose of COVID-19 vaccine up to four months after the first.”
This maximum of four months is a solid approach because either (1) the science is solid and net more protection is offered to the country, or (2) the science is just ok, but the supply and delivery improvements will actually allow for on time doses at a faster pace then previously possible