The AstraZeneca COVID-19 vaccine approved by EU and UK regulatory authorities used a meningitis vaccine as the control in phase 2/3 trials.

From the methods section of their paper:

Participants were randomly assigned to receive either the ChAdOx1 nCoV-19 vaccine or the quadrivalent MenACWY protein-polysaccharide conjugate vaccine. MenACWY was used as a comparator vaccine rather than a saline placebo to maintain masking of participants who had local or systemic reactions.

Confidence intervals for side effects overlapped between the treatment and control groups (see the appendix in the above linked paper), though the study wasn't powered to detect moderate differences. It seems these investigators may have met their goal of avoiding unmasking due to side effects.

This doesn't guarantee that unmasking didn't occur for some other reason. I'd note that no masking assessment was reported, though it rarely is. It is possible to assess masking by, e.g., asking participants whether they think they received the treatment or control. Unmasking can occur for a variety of reasons, side effects are just one of them. For example, trials of interventions that are a bit more complex than a pill (e.g., injection, IV) often don't blind staff who administer the intervention and control, but keep the participants, investigators, and the people who collect the data blind. This comes with a risk of unmasking, but is balanced against the challenge (and likely failure) of making the intervention and treatment look exactly alike, push exactly alike, etc.