CDC Says Overweight People Need Longer Needles for Vaccinations

If you were among the many who experienced a breakthrough SARS-CoV-2 infection despite getting a COVID-19 shot (or two or three), federal health officials suggest that—if you are overweight or obese—the length of the needle used to inject you might be to blame, not vaccine failure. New recommendations from the U.S. Centers for Disease Control and Prevention (CDC) suggest that certain people, especially those who are overweight or obese, may require longer needles for the shots to be effective.^{1 2}

The updated guidance comes more than two years after the initial rollout of the experimental messenger RNA (mRNA) shots and tells vaccine administrators that they may need to use a 1.5-inch needle and not the standard one-inch needle for some vaccines given to overweight people.³

The CDC explains that intramuscular shots for COVID, influenza and tetanus, for example, are injected into one of the deltoid muscles located toward the top of the shoulder. Because this muscle sits below a layer of skin, fat, and connective tissue that can vary in size from person to person, CDC officials have come to the conclusion that different needle sizes are needed for injecting vaccines into people with different body sizes.

CDC Says Needle Needs To Be Long Enough to Get Into Muscle But Not Hit Nerve, Bone or Blood Vessels

The guidance advises that intramuscular injections should be administered with needles that are long enough to reach a person’s muscle mass but not so long that they hit underlying nerves, bone, or blood vessels. The requirement differences in needle length vary depending on the type of vaccine and the person receiving it.¹

“With intramuscular [vaccines], you have to advance the needle beyond the subcutaneous layer,” said Matthew Laurens, MD, MPH, who leads studies at the University of Maryland’s Center for Vaccine Development. He added, “In addition to ensuring the vaccine is administered effectively, using the right needle length can also lessen the chances of adverse effects like swelling or redness at the vaccination site,” although he did not explain exactly why a longer needle that goes deeper into the body would not provoke a stronger, rather than a lesser, vaccine reaction. ¹

Wrong Needle Size Could Make Vaccines Less Effective, But Maybe Not

With the updated monovalent mRNA COVID shots becoming commercially available last month, some doctors suggest that people getting vaccinated should double-check to ensure that vaccine providers use the right-size needle for specific body types—speculating that the wrong needle length could decrease efficacy and offer less protection.

“If you recently received a COVID vaccine, and you suspect your provider didn’t use the right needle length for you, that doesn’t mean that your vaccine won’t still provide protection—though it might not provide as much protection as it could have,” said Amesh Adalja, MD, biosecurity and emerging infectious diseases expert at Johns Hopkins Bloomberg School of Public Health. “Vaccines given with the wrong needle length would likely still have some protection,” he added. “Even if some of [the vaccine] doesn’t reach your muscle, there’s likely still going to be an immune response to it. The way a vaccine is given, as well as the needle length used, can affect its efficacy.”^{1 4}

People Urged to Prompt Vaccine Providers to Use the Right Needle

While the updated guidance to use longer needles may benefit certain groups of people, experts worry it may not be implemented in medical settings, prompting a warning for patients to double-check with providers that the correct needle length is being used for their body type when receiving vaccines. The suggestion for patients to oversee their own medical care comes as no surprise as new data shows there is a 200 percent increase in medical errors among burnt-out physicians since the COVID pandemic. A staggering 63 percent of physicians are reporting symptoms of burnout, according to one study.^{5 6}

“I don’t know if we’re doing enough quality assurance to make sure we’re following these recommendations,” Dr. Laurens warned. “I think it’s great to be as informed as possible about the vaccine, including the length of the needle. The more informed we are, and the more we hold each other accountable, the better outcomes we have.”¹

“It’s plausible that some healthcare workers who administer vaccines could sometimes use the wrong needle length for people with bigger bodies,” Dr. Adalja said. “It may be that people become lax about that when they’re injecting. If you’re obese or overweight, it would be useful to ask the injector if that’s the correct needle length for your body size.”¹

The newest mRNA COVID shots—updated monovalent versions of Pfizer/BioNTech’s Comirnaty and Moderna/NIAID’s Spikevax—were approved by the U.S. Food and Drug Administration (FDA) last month. The CDC’s Advisory Committee on Immunization Practices (ACIP) signed off on the new shots the day after FDA’s approval, recommending them for children 12 years of age and older and releasing the products under Emergency Use Authorization (EUA) for children as young as six months.⁴

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