According to some experts, people have stopped aspirating [pulling back the plunger of the syringe] to check if they hit a blood vessel while giving intra-muscular injections. Unfortunately, many updated guidelines [from pre-pandemic days] suggest that ‘aspirating for blood return in the syringe’ is no longer necessary. This leaves a small chance of missing an inadvertent intravenous injection. In the past, aspirating was standard practice to ensure that the tip of the needle had not hit a blood vessel.

 If the needle tip enters a tiny blood vessel in this way, which happens only in the rarest of rare cases, the vaccine will directly enter the bloodstream and can potentially cause an acute reaction.

Thus, there is a greater chance of the adenoviruses being injected directly into the bloodstream.

This was originally proposed by Professor Neils Høiby from Denmark in March this year.

Study done on mice at the Munich University in Germany suggests that this rare complication in adenovirus vaccines could be occurring as a result of the vaccine being injected into the blood stream. Vaccine candidates of different varieties such as AstraZeneca (adenovirus-vector), Pfizer (mRNA) and a placebo were used to check for outcomes following different routes of injection.

The preprint of the study by Leo Nicolai and co-authors, posted on bioRxiv.org, highlighted accidental intravenous injection as a potential mechanism for post-vaccination thrombotic thrombocytopenic syndrome (TTS) also called vaccine-induced immune thrombotic thrombocytopenia (VITT).

“Our data provides first experimental support for the potential sequence of events that could lead to TTS in some patients: Thrombocytopenia might be a result of accidental intravenous adenovirus vaccine injection with ensuing platelet-adenovirus aggregation and activation with antibody formation. We recommend aspirating to check for blood in the syringe before injecting,” the authors said.

The risk of vaccine-induced clots is higher in women. The reasons for this are not fully known. Anatomically speaking, the needle is more likely to miss the deltoid muscle and hit the subcutaneous tissue in women.

Women have greater deltoid fat pad thickness - which means they have a thicker layer of fat separating the skin over the shoulder from the deltoid muscle underneath than men.
This means the needle has to penetrate deeper past the skin to “hit muscle” in women.

The same length needle is used for both men and women. This means that the chance of missing the muscle with the needle is higher in women. There is published evidence that the chance of the tip of the needle being in the subcutaneous tissue is three times higher in women.

Women may have other physiological risk factors for thrombosis too.

It has been observed that the risk of blood clots following vaccination is lower in the case of the second dose as compared to the first.

“It is about ten times less common with the second dose. One of the likely reasons for this is prior exposure [first dose] to the virus and hence a milder immune response even if it enters the bloodstream inadvertently. In the case of the first dose, the body has never been exposed to the virus.

CORRECT TECHNIQUE FOR INTRA-MUSCULAR INJECTION

1. Do not pinch skin up. Instead, gently stretch skin before plunging the needle at a 90 degree angle.
2. The needle tip needs to be in the muscle, not the subcutaneous tissue.
3. Aspirate gently before injecting. Make sure you haven’t hit a blood vessel.

Denmark’s Statens Serum Institut has already issued a special advisory regarding vaccination technique for all Covid-19 vaccines to prevent inadvertent injection into the bloodstream. The syringe has to be aspirated to check for blood before injecting the vaccine.

https://www.biorxiv.org/content/10.1101/2021.06.29.450356v1