CDC’s website, the second vaccine dose can be administered later than the recommended interval of time.The recommended time intervals between the first and second vaccine doses are:RELATED: CDC finds 10 cases of life-threatening reaction to COVID-19 vaccine out of more than 4M administered dosesBut the CDC has approved a longer wait for the second dose, stating it is safe and effective to administer the shot 6 weeks (42 days) after the first dose.AdvertisementThe CDC’s updated guidance also says that the Moderna and Pfizer vaccines can be interchanged between the first and second doses in "exceptional situations," but the agency stressed that such a scenario should be avoided and that vaccines should not be mixed, citing a lack of.