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Which is better: intramuscular or subcutaneous injections?

Yudai Kaneda

Hokkaido University School of Medicine

"Vaccination Error in 48 People: Nurse Mistakenly Gives Subcutaneous Injection Instead of Intramuscular"


A headline from late October reported a startling incident at the Shiga Prefecture Omi-Hachiman City General Medical Center. On October 13th, during a mass COVID-19 vaccination event, a nurse, mistaking the procedure for a flu shot, inadvertently administered subcutaneous injections to 48 individuals instead of the required intramuscular injections. While there have been no reports of health issues and no significant difference in efficacy, this error raises concerns about adherence to standard procedures.


At first glance, this might seem a minor issue – after all, what's the difference between intramuscular and subcutaneous injections? However, administering a vaccine in a way not specified in the product documentation can have serious implications, including adverse reactions for which medical professionals could be held accountable.


In Japan, due to past practices and a specific incident in the 1970s involving muscle contracture diseases in children from intramuscular injections of antibiotics and antipyretics, subcutaneous injections have been the norm for many inactivated vaccines, including the flu vaccine. This historical context has influenced current medical practices.


However, the COVID-19 vaccine is administered intramuscularly, and with over 80% of Japanese people having received two doses and nearly 70% three doses, intramuscular injections have become more common among healthcare workers. Furthermore, measures to combat COVID-19, such as social distancing and mask-wearing, have led to a significant decrease in influenza and other infectious diseases in Japan, with a reported 99% decrease from March 2020 to September 2021.


Yet, as of September 2023, influenza case reports have increased 166-fold compared to the previous year, underscoring the importance of flu vaccinations. Distinguishing between flu and COVID-19 based on clinical symptoms alone is challenging, making vaccinations crucial.


Given this context, the aforementioned incident highlights the confusion that can arise in medical settings when different injection methods are intermixed. So, which is better: intramuscular or subcutaneous injections?


The CDC and WHO recommend intramuscular injections for inactivated vaccines, including the flu vaccine. Studies show that intramuscular injections consistently yield better immunogenicity, fewer local reactions, enhanced antibody responses, and overall better outcomes. A study by Kameda General Hospital Group found that side effects were significantly lower with intramuscular (8.2%) compared to subcutaneous (11.3%) injections, with less pain reported as well.


Considering Japan's historical context, the prevailing practice of subcutaneous injections may have been inevitable. However, based on current evidence, it may be time to reconsider this approach, especially for the flu vaccine. A shift in vaccination methods requires revising documentation and conducting clinical trials, which involves time and cost. Yet, the lower immunogenicity and higher incidence of adverse reactions with subcutaneous injections could contribute to vaccine hesitancy, potentially disadvantaging patients.


With intramuscular COVID-19 vaccinations now common among healthcare workers, it's an opportune time to advance this discussion. I've recently published a paper on this topic, which I hope you'll find informative.

*This article is a translation of Japanese MRIC published on Dec 15, 2023


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