Blood in the Syringe. Is It Dangerous?

I've earned my RN license long ago and have administered thousands of intramuscular (IM) injections and vaccines.
In the past, when administering IM injections, aspirating was standard practice to ensure that the tip of the needle had not hit a blood vessel.

IM injection technique

Pay close attention to step 1 and 9

1. Locate injection site
To isolate the muscle and target where you’ll place the injection, spread the skin at the injection site between two fingers.
Read the atricle
Symptoms from improperly administered vaccinations - known as SIRVA, for "shoulder injury related to vaccine administration'' - include chronic pain, limited range of motion, nerve damage, frozen shoulder (the inability to move the shoulder) and rotator cuff tear.
Read a patient's story

2. Clean injection site
Clean the site selected for injection with an alcohol swab and allow the skin to air dry.

3. Prepare syringe with medication
Remove the cap. If the vial or pen is multi-dose, take a note about when the vial was first opened. The rubber stopper should be cleaned with an alcohol swab.
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4. Draw air into the syringe
Draw back the plunger to fill the syringe with air up to the dose that you’ll be injecting. This is done because the vial is a vacuum and you need to add an equal amount of air to regulate the pressure.

5. Insert air into the vial
Remove the cap from the needle and push the needle through the rubber stopper at the top of the vial. Inject all of the air into the vial. Be careful to not touch the needle to keep it clean.

6. Withdraw the medication
Turn the vial and syringe upside down so the needle points upward and pull back on the plunger to withdraw the correct amount of medication.

7. Remove air bubbles
Tap the syringe to push any bubbles to the top and gently depress the plunger to push the air bubbles out.

8. Insert the needle
Hold the needle like a dart and insert it into the muscle at a 90-degree angle. You should insert the needle in a quick, but controlled manner.
​Do not push the plunger in.
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9. Check for blood!
Pull back on the plunger slightly, looking for blood in the syringe. 
If you see blood going into the syringe, it means the tip of the needle is in a blood vessel. If this happens, withdraw the needle and begin again with a new needle, syringe with medication, and injection site.

What happens if you give an IM injection in a blood vessel?
"When injecting, ensure that no blood vessels are touched. If blood is visible in the syringe when pulling the syringe plunger back towards you, immediately remove the needle and press on the injection site to stop the bleeding. 
A broken blood vessel causes internal bleeding within the muscle. You may feel pain and stiffness in the muscle. When a blood vessel breaks, scar tissue or blood clots can form and if a blood clot starts to wander and reaches the heart or lungs, the consequences can be life-threatening.  Injections that hit an artery can be particularly dangerous."
Source

"Recent global studies suggest that wrong injection techniques used to administer Covid-19 vaccines may be a reason behind incidences of blood clots being reported post vaccination.
If the technique of administering the injection is wrong, the vaccine could get injected into the blood stream instead of reaching the muscle. This can lead to blood clots, health experts said."
Source

Updated guidelines

“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,” Dr Jayadevan said.​
Source

Knowledge is Power

Be safe!