My doctor said that flu shot was a must for fall and winter, but not spring and summer. ----- below is from some googled result: "You will get an RFE if your doctor didn't request a waiver for your flu shot because your medical examination happened outside of the flu season. You will also get an RFE if the medical examination happened during the flu season and you cannot show a flu shot in your records. Flu season is from October to March. Important is the date of the medical examination and not the date of submitting your I-485. You can find this answer on the USIC homepage under Q &A for vaccination requirements." ----- another counter-experience: Some doctor may mark the flu shot as "not age appropriate", and it seems that some cases like this are approved anyway.
Maybe it's okay. As some doctors marked the flu shot as "not age appropriate " and the 485 was approved anyway. Maybe each doctor's common practice is different. For some religious or special reasons, the doctors can help apply for some waiver. Just like the vaccines for pregnant women, some got their green cards without even doing those vaccines, some got RFEs. There's no definite rules.