In general, if the error is discovered on the same clinic day, you can administer the other “half” of the dose on that same day. If the error is discovered later, the dose should not be counted, and then the person should be recalled to the office and given a full age-appropriate repeat dose.
There are, however, two exceptions to the general rule: (1) If a patient sneezes after receiving nasal-spray live attenuated influenza vaccine, count the dose as valid. (2) If an infant regurgitates, spits, or vomits during or after receiving oral rotavirus vaccine, count the dose as valid.
If you give more than an age-appropriate dose, count the dose as valid and notify the patient/parent about the error. Using larger than recommended dosages can be hazardous because of excessive local or systemic concentrations of antigens or other vaccine constituents. Avoid such errors by checking the vaccine vial label 3 times.