My left upper arm itches constantly Could this be caused by Covid vaccine

The short version: constant itching in the left upper arm is a known possible reaction to the COVID-19 vaccine, and the evidence supports that.

The evidence you have includes a case report of a 48-year-old woman who developed itching (pruritus) at the injection site on her left deltoid within 10 hours of the second Sputnik-V dose [2]. The itching lasted about a day. A larger study from Brazzaville found that about 4 in 100 people who reported any adverse event after COVID-19 vaccination reported itching or rash [1]. That study covered Sinopharm, Sputnik V, and other vaccines. A narrative review of skin reactions to COVID-19 vaccines also lists itching as a known local reaction, with rates varying by vaccine type [3]. For the mRNA vaccines, one study found that about 0.8% of people reported a delayed local reaction (sometimes called "COVID arm") after the first dose, and 0.2% after the second [3]. That reaction typically starts around 7 days after the shot and can include itching, redness, and swelling.

The mechanism is straightforward. The vaccine triggers an immune response at the injection site. That response involves inflammation, and inflammation causes itching. The lipid nanoparticles in mRNA vaccines, the viral vector in adenovirus vaccines, and the adjuvants in other types all provoke local immune activation. For some people, that activation is strong enough to produce a persistent itch. The itching can last days or weeks, and in some cases it becomes a chronic localized reaction.

The evidence here does not include a long-term study of people with persistent injection-site itching. The case report shows it resolved within a day [2]. The Brazzaville study found that about 8% of people who reported any adverse event had symptoms lasting more than a week [1]. That could include itching. But the study did not track itching separately over time. So the question of whether constant itching that lasts months is caused by the vaccine is not answered by these records. It is plausible on mechanism: the immune response can persist locally. But the evidence does not establish a rate or a typical duration.

My call: constant itching in the left upper arm can be caused by the COVID-19 vaccine. The evidence shows it happens, and the mechanism is clear. But the evidence does not tell you how long it typically lasts or how common persistent itching is. Confidence: moderate for the association, low for the duration and frequency of persistent cases.