Will an influenza (flu) vaccine be effective if administered into the subdeltoid bursa?

From the Guidelines

A flu shot administered into the subdeltoid bursa can still be effective, though it's not the ideal location, and the most recent guidelines from 2013 recommend intramuscular administration in the deltoid muscle for adults and older children 1. The flu vaccine is designed to be given intramuscularly, typically into the deltoid muscle. When the vaccine enters the subdeltoid bursa (a fluid-filled sac between the deltoid muscle and shoulder bone), it can still be absorbed into the bloodstream and trigger an immune response, but the absorption may be slower or less complete than with proper muscle injection. This suboptimal placement might potentially result in reduced vaccine effectiveness or increased local side effects like pain and swelling. The immune system will still recognize the vaccine antigens regardless of the injection site, but the timing and strength of the immune response could be affected. Some key points to consider include:

• The intramuscular route is recommended for influenza vaccine, with the deltoid muscle being the preferred site for adults and older children 2, 1, 3, 4.
• A needle length >1 inch can be considered for these age groups because needles <1 inch might be of insufficient length to penetrate muscle tissue in certain adults and older children 3, 4. If you suspect your flu shot was administered into the bursa rather than the muscle, you generally don't need a repeat vaccination, but you should mention this to your healthcare provider for proper documentation and to ensure correct technique for future vaccinations. It's also important to note that the most recent and highest quality study, from 2013, provides the best guidance on this topic, and its recommendations should be prioritized 1.

From the Research

Effectiveness of the Flu Shot in the Subdeltoid Bursa

• The flu shot's effectiveness when placed in the subdeltoid bursa is not directly addressed in the provided studies, which focus on the complications and risks associated with vaccine administration in this area 5, 6, 7, 8, 9.
• However, it can be inferred that the vaccine's efficacy may not be significantly impacted by the injection site, as the primary concern with subdeltoid bursa injections is the risk of shoulder injury and bursitis, rather than the vaccine's effectiveness 6, 7, 8.

Risks Associated with Subdeltoid Bursa Injections

• Studies have reported cases of subdeltoid bursitis, shoulder pain, and weakness following influenza vaccine injections into the subdeltoid bursa 5, 6, 7.
• The risk of subdeltoid bursitis after influenza vaccination is estimated to be small, with an attributable risk of 7.78 additional cases per 1 million persons vaccinated 9.
• Improper administration of the vaccine is a primary reason for injection-related subdeltoid bursitis, highlighting the importance of proper training for healthcare workers 7, 8.

Proper Administration Techniques

• An evidence-based protocol for safe vaccine administration into the deltoid muscle recommends injecting at the midpoint between the acromion and the deltoid tuberosity, with the arm abducted to 60° 8.
• This technique can help minimize the risk of upper arm injury related to vaccine administration (UAIRVA) and subdeltoid bursitis 8.