Not sure if the BMJ study from last March was discussed here but looks like there's a follow up study referenced in the NYTimes today:
Without going too deep, basically this (not-yet-peer-review-pre-print) study the NYTimes references found that: "34 percent of people taking the drug [Remicade] were protected after a single dose of the Pfizer vaccine and only 27 percent after a single dose of the AstraZeneca vaccine."
But SECOND DOSE of vaccine led to seroconversion in most patients. Which feels like good news to me. The implication being not to delay your second dose.
Disclaimer: This is just one study done and it's not been peer-reviewed yet. Also, we should be happy that those of us on infliximab are being studied in relation to this virus and these vaccines. Always good to keep informed on your health and the medicines you take. If you're here, you're probably already doing that