Life You have a lump on your arm weeks after monkeypox vaccine. Why it’s a ‘super common’ side effect. August 23, 2022August 25, 2022Patrick Ryan, USA TODAYComments Off on You have a lump on your arm weeks after monkeypox vaccine. Why it’s a ‘super common’ side effect. One common complaint of monkeypox vaccination is a hard red bump at the injection site. But there's no need to worry about it, experts say.