According to health organizations, the flu vaccine is safe during pregnancy in any trimester, including the second one you're in. The vaccine used for pregnant women is the inactivated virus version (not the live virus one), which makes it safe for you and also protects your baby after birth because you pass the antibodies to them. Your doctor is right in recommending it before winter, so trust her judgment more than what your friends say without medical sources. Have you had other vaccines during pregnancy already, or is this the first one they're recommending?