What type of adverse reaction is most likely to occur after intranasal influenza vaccination?

The most likely adverse reaction after intranasal influenza vaccination is a mild systemic reaction. This type of vaccine administration is designed to stimulate an immune response, and as a result, some recipients might experience mild symptoms that can include fever, headache, or muscle aches. These reactions are typically transient and indicate that the immune system is responding to the vaccine.

In contrast to severe allergic reactions, which are rare and usually associated with certain components of a vaccine, mild systemic reactions are more common and expected after many vaccines, especially live attenuated ones like the intranasal influenza vaccine. Local reactions such as redness or swelling at the site are more associated with injectable vaccines and are less relevant to the intranasal route, while the option indicating no reaction at all does not consider the normal immune responses that can occur after vaccination. Thus, a mild systemic reaction is the expected outcome after administering the intranasal influenza vaccine.