Administering a flu shot is a straightforward intramuscular injection into the deltoid muscle, but each step in the process matters for safety, comfort, and vaccine effectiveness. From verifying the vaccine has been stored at the right temperature to choosing the correct needle length and placing the injection at a precise spot on the upper arm, small details can mean the difference between a routine vaccination and a complication like a shoulder injury or a dose that never reaches the muscle. Here is what the full process looks like, and why each piece of it exists.
Screening Before You Draw Up the Dose
Before any needle comes out of a drawer, the person receiving the vaccine needs a quick screening. The goal is to catch contraindications and precautions: a history of severe allergic reaction to a previous flu vaccine or one of its components, a current moderate-to-severe illness with fever, or, for certain live attenuated formulations, pregnancy or immunosuppression. The Immunization Action Coalition publishes a standardized screening checklist widely used by occupational health nurses and pharmacists to walk through these questions systematically and flag problems before they become adverse events.1PubMed. Assessing Vaccine Contraindications Through the Use of the Immunization Action Coalition Screening Checklist
The screening also includes asking about egg allergy, though this has become less of a concern in recent years. Cell-based and recombinant flu vaccines contain no egg protein at all, and even egg-based vaccines contain such trace amounts that the CDC no longer considers egg allergy a reason to avoid them for most people. Still, asking the question helps identify anyone with a history of anaphylaxis who should be observed for a longer period after the injection.
Checking the Vaccine Itself
Flu vaccines are biological products that lose potency when exposed to temperatures outside their recommended storage range of 2–8°C. This is not a theoretical concern. Research monitoring cold chain devices across multiple countries has documented that temperature excursions beyond that range occur at a troubling rate. One systematic review of cold chain logistics in India found that temperatures strayed outside the recommended window at somewhere between a third and nearly three-quarters of cold chain points, depending on the state.2International Journal of Drug Delivery Technology. Cold Chain Logistics Challenges In Vaccine Distribution In India: A Systematic Review Freezing is actually the more insidious problem: vaccines exposed to sub-zero temperatures for extended periods can lose potency in ways that are invisible to routine manual temperature monitoring and not caught by standard vaccine vial monitors.3PubMed Central. Temperature integrity and exposure of vaccines to suboptimal temperatures in cold chain devices at different levels in three states of India
A 2024 cross-sectional study tracking vaccine shipments in Ukraine found recorded temperatures ranging from −13.6°C to +28.4°C along distribution routes, with compliance varying widely by facility.4PubMed Central. Assessing vaccine cold chain storage and transport in Ukraine: a cross-sectional study The practical takeaway for the person giving the shot: always verify the storage unit’s temperature log, check the vaccine vial monitor if one is present, inspect the liquid for unusual particles or discoloration, and confirm the expiration date. A flu vaccine that has been frozen and thawed should not be administered.
Choosing the Right Needle
Flu shots for adults are intramuscular injections, meaning the needle needs to pass through the skin and the layer of subcutaneous fat to deposit the vaccine into the deltoid muscle itself. If the needle is too short, the vaccine ends up in fat tissue, where absorption is slower and the immune response can be weaker. If it is too long, there is a greater risk of hitting deeper structures. Current CDC guidelines base needle length recommendations on the recipient’s weight and sex, because subcutaneous fat thickness over the deltoid varies considerably between individuals.5PubMed Central. Statistical estimation of deltoid subcutaneous fat pad thickness: implications for needle length for vaccination
For most average-sized adults, a 1-inch (25 mm) needle with a 22- to 25-gauge bore is standard. Larger adults may need a 1.5-inch needle. Smaller adults or adolescents can often use a 5/8-inch needle, though only if the deltoid can be adequately bunched to ensure intramuscular delivery. For children under three, the anterolateral thigh rather than the deltoid is the preferred site, and needle length drops accordingly. The gauge number refers to the needle’s diameter: higher gauge means thinner, which generally means less pain on insertion but slower delivery of the fluid.
Finding the Right Spot on the Deltoid
This is where many injection errors originate, and where the consequences can be most serious. The deltoid muscle sits like a cap on the top of the shoulder, originating at the acromion (the bony point at the tip of the shoulder) and attaching partway down the upper arm. The safe zone for injection is not at the top of the muscle near the shoulder tip. It is lower than most people assume.
A cadaveric study measuring the positions of the axillary nerve and the posterior circumflex humeral artery found that both structures sit in the upper half of the deltoid, roughly 8 to 9 cm below the acromion. The muscle itself is thickest between 7 and 8 cm from its origin. The study’s authors recommend placing the injection about four fingerbreadths, or approximately 9 cm, below the mid-acromion point to stay safely below the nerve and artery while still hitting the thickest part of the muscle.6PubMed Central. The Recommended Deltoid Intramuscular Injection Sites in the Adult Population: A Cadaveric Pilot Study With an Orthopedic Perspective A complementary guideline from pharmacy practice literature describes the target as two to three fingerbreadths below the acromion, within an inverted triangle on the lateral aspect of the arm.7PubMed Central. Getting it in the right spot: Shoulder injury related to vaccine administration (SIRVA) and other injection site events The slight difference in landmarks reflects the fact that fingerbreadth measurements vary by hand size, but both guidelines converge on the same anatomical zone: the middle third of the muscle, well below the shoulder tip.
Getting this wrong by even a couple of centimeters too high can lead to a condition called shoulder injury related to vaccine administration, or SIRVA. This occurs when the needle enters the subdeltoid bursa, a fluid-filled sac that cushions the shoulder joint, rather than the muscle. The result can be severe shoulder pain, inflammation, and dysfunction lasting weeks to months.7PubMed Central. Getting it in the right spot: Shoulder injury related to vaccine administration (SIRVA) and other injection site events SIRVA is not caused by the vaccine’s contents; it is purely a mechanical injury from incorrect placement.
Positioning the Arm
How the recipient holds their arm during the injection affects both the ease of finding the right spot and the risk of complications. The cadaveric study mentioned earlier recommended having the recipient place their hand on their hip, which abducts the shoulder to about 60 degrees. This position pulls the deltoid taut, making the muscle’s midpoint easier to palpate, and shifts the neurovascular bundle slightly, adding a margin of safety.6PubMed Central. The Recommended Deltoid Intramuscular Injection Sites in the Adult Population: A Cadaveric Pilot Study With an Orthopedic Perspective
In practice, many vaccine recipients simply let their arm hang at their side. This is acceptable as long as the person relaxes the muscle. A tensed deltoid is harder to inject into and may increase post-injection soreness. Asking the recipient to drop their shoulder and let the arm go limp, or to place the hand on the opposite hip, accomplishes muscle relaxation while keeping the injection site accessible. For seated recipients in a pharmacy or clinic, the arm should rest naturally on the lap or armrest of the chair, with the sleeve pulled well above the injection zone so the vaccinator can see the acromion clearly.
Performing the Injection
With the site identified and the arm positioned, the actual injection follows a quick sequence. Clean the skin at the injection site with an alcohol swab using a firm circular motion, then let it air-dry for a few seconds. Injecting through wet alcohol can sting and theoretically could carry surface contaminants into the tissue, though the real-world infection risk from vaccination is extremely low regardless.
Hold the syringe like a dart, bevel up, and insert the needle at a 90-degree angle to the skin surface in a single smooth motion. For lean adults, you can lightly pinch or bunch the muscle to provide more tissue depth; for average or larger adults, stretching the skin flat with the non-dominant hand is typically sufficient. Push the plunger steadily and evenly. Rapid injection forces the fluid into a smaller space and can increase soreness. Once all the vaccine has been delivered, withdraw the needle at the same angle it went in and apply gentle pressure with a clean cotton ball or gauze.
The Aspiration Question
If you trained as a nurse or medic more than a decade ago, you were likely taught to pull back on the plunger after inserting the needle and check for blood before injecting. The idea was to confirm you had not hit a blood vessel. This practice has largely been abandoned for vaccinations. A meta-analysis examining pain, procedure time, and clinical outcomes found that aspiration increases pain and lengthens the procedure without reducing complication risk.8PubMed. Aspiration in intramuscular injection: a meta-analysis of pain, duration, and clinical outcomes International guidelines now discourage routine aspiration for intramuscular vaccine injections, and the CDC does not recommend it.9PubMed Central. To aspirate or not to aspirate? Considerations for the COVID-19 vaccines The deltoid simply does not contain large blood vessels in the injection zone, and the few seconds of needle movement during aspiration cause unnecessary discomfort.
Prefilled Syringes Versus Multi-Dose Vials
Most flu vaccines in pharmacies and clinics arrive as prefilled single-dose syringes. You remove the cap, attach the needle, and inject. The dose is already measured, which eliminates drawing errors and reduces contamination risk. Multi-dose vials, which contain enough vaccine for multiple patients, are still used in high-volume settings like mass vaccination clinics. They require the vaccinator to draw each dose individually using a new needle and syringe for each patient.
Multi-dose vials introduce a few extra considerations. Each time a needle punctures the vial’s rubber stopper, a small amount of vaccine can be lost in the hub of the needle and syringe. Research on techniques to maximize the number of doses from a vial found that an air-bubble-trapping method produced comparable volumes to conventional drawing but took roughly twice as long to prepare, around 30 to 33 seconds per dose compared with 12 to 15 seconds.10PubMed Central. Maximizing doses from multi-dose vaccine vials using the air bubble trapping technique In a mass vaccination setting where every minute counts, that added time matters. Most vaccinators accept the small volume loss and work with the standard drawing technique, discarding the vial when they can no longer withdraw a full dose.
A key rule with multi-dose vials: never pre-draw doses and leave them sitting in syringes for extended periods. Once drawn, a dose should be administered promptly. And the vial itself must be discarded at the end of the clinic day or according to the manufacturer’s beyond-use dating, whichever comes first.
Reducing Pain and Anxiety
Needle anxiety is a real barrier to vaccination. Some estimates put the proportion of adults who avoid vaccination partly because of needle fear in the range of one in six to one in four. A few evidence-based techniques can make the experience better.
For infants and young children, devices that combine vibration and external cold applied near the injection site have been shown to reduce both pain intensity and crying duration. In one randomized trial of 80 infants, the group receiving vibration-plus-cold had lower mean pain scores and cried for a shorter time compared with the control group.11PubMed Central. The Effect of a Combination of Vibration and External Cold on Pain Caused during Vaccine Injection in Infants: A Randomized Clinical Trial Commercially available devices using this principle (often called “buzzy” devices) are inexpensive and increasingly common in pediatric clinics.
For adults, the most effective pain-reduction strategies are simpler:
- Relaxed muscle: A tense arm hurts more. Remind the recipient to let their arm hang loose.
- Smooth insertion: A quick, confident needle entry is less painful than a slow or hesitant one.
- Distraction: Asking the person a question or having them look away during insertion reduces perceived pain.
- Slow plunger push: Injecting the fluid steadily over a couple of seconds rather than in a sharp burst reduces tissue pressure.
- Thinner needle: Where gauge options exist, a 25-gauge needle causes less sensation than a 22-gauge, though it takes slightly longer to push the fluid through.
Rubbing or massaging the site immediately after injection is sometimes done but is not strongly supported by evidence. It may help distribute the vaccine in the tissue and reduce the feeling of a lump, but it can also increase bruising in people who bruise easily.
What Happens Right After the Shot
Standard practice is to observe vaccine recipients for at least 15 minutes after administration, mainly to watch for anaphylaxis or vasovagal syncope (fainting). Anaphylaxis following a flu vaccine is rare but happens fast, usually within minutes. Every vaccination setting should have epinephrine readily available.
Fainting is more common than anaphylaxis, especially among adolescents and young adults, and it can cause injury from a fall. The vasovagal response involves a sudden drop in heart rate and blood pressure triggered by the stress of the injection itself, not by the vaccine’s contents. It typically begins with lightheadedness, nausea, warmth, and visual dimming.12PubMed. Neural circulatory control in vasovagal syncope Having the recipient remain seated, keeping them hydrated, and engaging in conversation during the observation window are all practical measures. If someone begins feeling faint, having them lie down with legs elevated usually resolves the episode within minutes.
After leaving the vaccination site, recipients should be told that arm soreness, mild swelling, and low-grade fever over the next day or two are normal immune responses. These are not signs that something went wrong; they are signs the immune system is responding. Heavy exercise with the injected arm on the same day is fine for most people, though some prefer to rest it.
High-Dose and Adjuvanted Formulations for Older Adults
Not all flu shots are administered the same way because not all flu shots are the same product. Adults 65 and older are typically offered high-dose or adjuvanted formulations specifically designed to overcome the weaker immune response that comes with aging. The injection technique itself does not change, but the vaccine inside the syringe is different.
High-dose flu vaccine contains four times the antigen of a standard dose. A systematic review and meta-analysis found that compared with standard-dose vaccine, the high-dose version reduced influenza-like illness by about 21% in adults aged 65–74 and by about 25% in those 75 and older. The benefit was even more pronounced for hospitalizations: roughly 9% relative effectiveness in the 65–74 group, climbing to about 16% in those 85 and older.13PubMed Central. High-dose influenza vaccine in older adults by age and seasonal characteristics: Systematic review and meta-analysis update The trend of increasing benefit with increasing age makes sense: the oldest adults have the weakest baseline immune response and therefore gain the most from a stronger antigenic stimulus.
Adjuvanted vaccines take a different approach, including a compound that amplifies the immune response rather than simply adding more antigen. Both types cause slightly more arm soreness and mild systemic reactions than standard-dose vaccines, but serious adverse events are no more common. The vaccinator should let the recipient know to expect a sore arm that may be a bit worse than in previous years if it is their first time receiving one of these enhanced formulations.
When the Flu Shot Is Not an Injection at All
The nasal spray flu vaccine, a live attenuated formulation, bypasses the entire intramuscular injection process. It is sprayed into the nostrils as a mist and triggers an immune response in the mucosal lining of the respiratory tract. It is approved for non-pregnant individuals aged 2 through 49 without certain underlying health conditions. For people who genuinely cannot tolerate needles, it offers a legitimate alternative, though it is not recommended for adults 50 and older, pregnant individuals, or those with weakened immune systems.
The administration technique is simple: the recipient sits upright, the vaccinator inserts the tip of the sprayer just inside one nostril, depresses the plunger to deliver half the dose, then repeats in the other nostril. No aspiration, no landmarking, no needle-length selection. The recipient may sneeze afterward, which does not reduce the vaccine’s effectiveness. One practical note: if the recipient has significant nasal congestion at the time of vaccination, the mist may not coat the mucosal surfaces effectively, and some clinicians prefer to reschedule or offer the injectable version instead.