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Pfizer's new coronavirus vaccine can only be collected in 5 doses with a conventional syringe, but it is said that 6 doses can be collected with a special syringe called the loaded space type. Why does the number of inoculations differ depending on the type of syringe?

The stock solution of one vial of Pfizer's new coronavirus vaccine is 0.45 mL, but it is 2.25 mL because it is diluted by adding 1.8 mL of physiological saline. Since the vaccine to be administered to one person in one inoculation is 0.3 mL, it can be collected 7 times in calculation, but when using a general syringe, a part of the drug solution remains on the barrel tip due to the structure of the syringe. Actually, it sucks up 0.3mL or more. The part of the gap where the chemical solution remains is called the dead space, and the chemical solution that remains there does not come out even if it is pushed in. When the drug solution is sucked out with a syringe, it is said that the 0.3 mL required for one inoculation does not remain in the sixth inoculation. The Ministry of Health, Labor and Welfare also admits that the syringes normally used in Japan can only collect 5 doses. The drug solution remaining in the syringe will be discarded.

A loaded space type syringe was developed to prevent this loss of chemicals. Since the outer cylinder and the tip of the pusher (plunger) are precisely aligned, it is possible to push out the chemical solution remaining on the cylinder tip to the maximum. With this type of syringe, it is possible to reduce the chemical loss remaining in the gap at the tip of the syringe to 0.035 mL or less, and the chemical loss is about 95% less than when using a general syringe, 6 It is possible to collect batches (Fig.).

(Figure) Dead space structure of general syringes and loaded space type syringes

By this mechanism, the fact that 5 doses are increased to 6 doses despite the same amount means that the number of inoculations will increase by about 20%. In addition, it can be used without wasting expensive chemicals.

Loaded space type syringes are limited to insulin administration and are used less frequently than commonly used syringes, so they are in short supply worldwide. Even in Japan, there is no prospect of securing a sufficient number. The Ministry of Health, Labor and Welfare has recently decided to reduce the number of inoculations per vial from the originally expected 6 to 5 because it is difficult to secure a special syringe. A system to secure solid medical equipment is desired so as not to waste valuable vaccines.