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In case in this pandemic that we are having to suffer we had not had enough to have to become experts in disinfectant gels, masks and interior ventilation, now comes the dilemma of the syringes to be able to administer the coronavirus vaccine.

What seemed like it was never going to run out is suffering constant stock outs

, especially in some autonomous communities, and the possibility of extracting an extra dose from each vial of Pfizer's vaccine has

out of stock of a certain model of syringes.

That extra dose that hasn't been talked about until now

It has nothing to do with the miracle of the multiplication of loaves and fishes, it has a much simpler explanation that obeys mathematics and the use of a certain model of syringe that has been rare until now.

I invite you to continue reading and discover with me the "trick" behind the sixth dose.

How many doses can be obtained with each vial?

In principle it was said that 5



Pfizer vaccine vial

against the coronavirus, once thawed, we see that it has 0.45 ml of concentrated vaccine inside.

Nurses must then add 1.8 ml of 0.9% saline solution to the vial, mixing it with caution.According to the leaflet of the vaccine itself, from that mixture we must extract 5 doses of 0.3 ml, which is the amount that we will inject each person.

Where does the possibility of the sixth dose come from?

After extracting these 5 doses, we could see that, at the bottom of each vial, there was a small amount of excess vaccine that was thrown away.

That small amount was not enough to obtain a full dose (0.3 ml) ... unless we used what we know as

"no residue" or "low dead volume" syringes

, in which case we could get that final full dose.

But we lacked the authorization of the pharmaceutical company and the European Medicines Agency (EMA) to be able to do so.

When is that sixth dose authorized?


January the 8th


On that day, the EMA accepted a change in the Pfizer vaccine insert stating that, from each vial, 6 doses can be withdrawn instead of 5 if you have the right syringes.

What's so special about these "no residue syringes"?

The difference is in the end of the plunger.

In all syringes there is a dead space between the needle and the syringe plunger known as a "cone".

When we inject something, a very small part of that drug remains in the cone and is not injected, since that space is not occupied by the plunger as it is wider than the cone.

A very small fraction of medicine or vaccine that is lost, something that is generally unimportant.

"no residue or low dead volume"

For their part, they are syringes with the end end of the elongated and narrow plunger so that, when the medicine is injected, it penetrates the dead space of the cone, occupying it and preventing the vaccine from being lost and thrown away.

Are there enough syringes of this type?

Some autonomous communities have acknowledged having supply problems.

They were unusual syringes until now in daily practice

, and its demand has increased as soon as that sixth dose has been authorized.

Does this allow 6 doses to be taken out of all vials?

No, but yes from almost everyone


In order to always get that extra dose, the most important thing is to have the right equipment, but skill and luck also play a role when reconstituting the vial, purging the air and extracting the doses.

Any small problem can cause a drop or two to be missed and it would be enough to miss that sixth dose.

Could you take a little out of several vials to get a dose?

No, in no case


Doing so would pose a significant risk, since we would be mixing different batches and different vials.

In the event that the pharmacist notified the withdrawal of a batch, we would not have control over that dose, and in the event of an allergic reaction we could not link it to a specific batch. If the sixth dose cannot be removed from a vial, it is thrown away.

Is that sixth dose as effective as the others?


We do not take out 6 vaccines now because we are reducing the dose of each immunized person, or because we prepare the vial with more saline solution and it remains "watery".

We do everything exactly the same as when it was 5 doses

, but it can be said that now we are more meticulous with each drop of the vial.

Will we have more vaccines in Spain than planned thanks to this extra dose?

I'm afraid not.

The pharmaceutical company Pfizer has already warned that

the agreements it has reached with the European Union speak of doses, not vials


Europe has purchased 600 million doses from Pfizer, and

Spain has 60 million


The positive part is that we can vaccinate a little faster thanks to that extra dose, because with each vial we immunize six people.

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