Life You have a lump on your arm weeks after monkeypox vaccine. Why it’s a ‘super common’ side effect. August 23, 2022August 24, 2022 Patrick Ryan, USA TODAY Comments 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.