Canadian government switches to support for first doses first | Hacker News Reader

Canadian government switches to support for first doses first

canada.ca

16 pointsepivosism6 comments

The summary:

    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.

So basically, it makes theoretical sense, looks beneficial in total, so let's consciously adjust the vaccine administration regime we studied.

I've been fairly disappointed with the analysis of first doses first. If someone were to recommend half-courses of antibiotics, I feel like a lot more people would instantly recognize what could potentially go wrong here (and for which we will have 'no evidence for' until it is too late). The assumption of first doses first is that partial protection is better than all or none - but with evolving organisms, this is a pretty tendentious assumption. It is not at all implausible that this strategy will select for vaccine-immune strains.

Is there something in NACI's analysis that you disagree with specifically?

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.

Would combining two people's double doses all into one person, raising their resistance even more while leaving three people uncovered be beneficial in not setting up situations that would select for vaccine-immune strains? This is implied by your argument in favor of maximizing concentrated immunity rather than maximizing total population immunity.

i.e. if you think 1x0%, 1x95% is better than 2x85%, do you not also think that 3x0%, 1x99% is better?

Your question is addressed here: https://dash.harvard.edu/bitstream/handle/1/37366988/Dose_sp...

  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.

The better argument for first doses first is the expectation that vaccine supply and delivery will rise significantly in the near term. This would allow a country to start everyone’s vaccination timeframe sooner with a strong expectation that supply would bring second doses closer to the normal timeframe over time.

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

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