Safety of COVID-19 vaccines: Reports of delayed local reactions

Swissmedic is receiving reports of delayed local reactions occurring around the injection site following a vaccination to protect against COVID-19. Most of the reports to date involve the COVID-19 vaccine from Moderna, which has been authorised in Switzerland since 12 January 2021. The reports of redness and swelling predominantly occur around one week after the vaccination and have also been observed in other countries (so-called "COVID arm"). According to the latest findings, this is a temporary and harmless reaction connected with the activation of the body's immune system that disappears again after a few days.

These delayed local reactions can occur around one week after the vaccination and usually manifest themselves as a well-defined area of red, swollen skin on the injected arm, in some cases accompanied by pain and/or itching. These reactions improve without further measures after a few days.

In the authorisation study for the COVID-19 vaccine from Moderna, such reactions were observed in 0.8% of vaccinated subjects after the first dose and in 0.2% after the second dose. Accordingly, their frequency is currently classed as "uncommon" (≥ 1/1,000 to < 1/100). In the USA these reactions have also been termed "COVID arm" in the media.

In a number of the reports to Swissmedic it appears that some people with reactions that were retrospectively considered to be probable delayed local reactions had been treated with an antibiotic, based on the presumably incorrect suspicion of a bacterial skin infection (erysipelas).

The exact mechanism underlying these reactions is not known. The interval corresponds to the first occurrence of the antibodies and immune cells induced by the vaccination. In severe cases, treatment with analgesics or antihistamines can be considered.

Do not skip second vaccine dose
In some cases, there is uncertainty as to whether patients with a delayed local skin reaction should receive the second dose of the vaccine. The second dose is important for effective vaccine protection. There is no reason to skip or delay the second dose in the individuals concerned.
Some authors recommend injecting the second dose of the vaccine in the other arm.