What’s the chance symptoms last more than 3 days after the shot?

The short version: the studies here don't give a clean probability, but most symptoms that last past 3 days are the unusual ones, and the honest answer is that nobody has measured this properly for the general population.

The one study that directly reports duration is from India: among 448 people, about 12 in 100 had symptoms lasting under a day, about 36 in 100 lasted 1–3 days, about 2 in 100 lasted 4–7 days, and under 1 in 100 lasted more than a week [3]. That puts the chance of symptoms past 3 days at roughly 3 in 100 people who got a COVID shot. But this was a single hospital-based survey in West Bengal, not a randomized trial, and it only covered Covaxin and Covishield [3].

The other studies don't give you a duration. A Sri Lankan study found symptoms in about half of people after the first dose and about 6 in 10 after the third, but never says how long they lasted [1]. A children's arthritis study said tiredness, headache, and fever resolved within 1–2 days [5]. A shoulder-pain study found most pain ended within 24 hours to 7 days, with about 7 in 100 reporting pain lasting 6–7 days after the first dose [4]. A few unusual events lasted longer: hand weakness for 13 days, chest pain averaging 6 days [2].

What's missing: no study here tracks symptoms past 3 days in a general population with modern mRNA vaccines, and none covers flu shots given alongside COVID shots. The 3 in 100 figure is a rough estimate from one survey, not a proven rate.

My call: expect roughly 3 in 100 people to still feel symptoms past day 3 after a COVID shot, but the evidence is too thin to call it a firm probability. Confidence: low.