Thread by @WilsonKM2

Quick thread on 2nd doses and vaccine roll-out (and why my vaccine graph from yesterday accidentally went to the future).

You've probably seen headlines like the one below saying that Nova Scotia will soon be out of vaccine.

But then heard we have ~14k in storage. What gives?

I'll use two provinces: Nova Scotia (holds 100% of 2nd doses in reserve) and Ontario (does not).

Top/solid line are doses distributed to the province, bottom/dashed line are doses administered, and the shaded area are doses in storage.

NS has more in storage, proportionately

Both of the currently authorized vaccines in Canada require a 2nd dose (Pfizer at 21 days, Moderna at 28).

Provinces have taken different approaches to dealing with 2nd doses.

The most aggressive is Quebec, which is delaying 2nd doses out to 90 days in favour of giving more (v]vulnerable) people their first doses.

The middle ground is most of the other provinces, where they using most of their vaccines as they come in the door for a mix of 1st and 2nd doses as needed, trying to keep the 2nd doses in window*

*NACI has advised pushing the window out to 6 weeks should be fine.

And then there's reserving doses in advance. You vaccinate someone and then put their 2nd dose in the freezer for 3-4 weeks until they come back.

This upside of this approach is that it guards against severe supply chain disruption. You always have enough for the 2nd dose even if you never receive another dose.

The downside is that it is slower. We're in a pandemic and speed matters.

So let's see what holding doses in reserve looks like.

Here are the graphs from before, with the red-shaded area being doses that would be held in reserve. Every time someone gets their 1st dose, one goes in reserve. When they get a 2nd dose, it exits reserve.

Some things you may have noticed:

NS is *almost* out of vaccine if you consider the reserve doses off limits. Or it has a glut of unused doses if you think they should prioritize getting doses in arms over

Also: Ontario is not holding back doses and so is in a deficit, so to speak.

Ontario 'owes' more 2nd doses in the future than it currently has on hand.

Oh course, you don't 'owe' the 2nd dose at the moment you give the first. You only need it... at the time of the 2nd dose.

That's 21 days for Pfizer/28 for Moderna if we're being strict, or 6 weeks if we're not and just following NACI recommendations.

Here's NS if we shift 2nd dose obligations out 21 days (lighter red) or 6 weeks (darker red).

If NS never sees another dose, it can cover all it's 2nd doses still needed and *still* have some left over.

That's not the case for Ontario. They currently would have to use 100% of newly received doses to cover 2nd dose obligations (if they're sticking to the 21/28 day windows).

If they're using the broader six-week window, they have a lot more wiggle room.

NS is betting/acting like they will *never* be shipped another dose and therefore going slow to avoid delaying 2nd doses.

Ontario is betting/acting like they *will* get more doses and so is using that they have at the risk of delaying 2nd doses if that doesn't happen.

For what it's worth, neither is perfectly true.

There have been/will be supply disruptions that we'll have to plan around/be flexible for.

And also, being slower than you can be on vaccinations in the middle of a pandemic is not great.

If you're inclined, you can read a great paper on the topic here: acpjournals.org/doi/10.7326/m2…