how long until covid vaccine is effective

COVID-19 vaccines typically start to be effective about 7 to 14 days after vaccination, with significant protection developing after the second dose in a primary series. For example, studies show that Pfizer/BioNTech and Moderna vaccines reach high effectiveness approximately two weeks after the second dose, with initial protection observed about 15 to 28 days after the first dose. Vaccine effectiveness is highest in the first few months following vaccination but can wane over time, requiring booster doses for maintained protection. The 2024–2025 vaccines showed effectiveness within the first 7 to 119 days post-vaccination, such as 33% effectiveness against emergency visits and around 45% effectiveness against hospitalization in older adults during this period. Timing of vaccination during the day may even influence effectiveness, with some data suggesting midday vaccinations might provide slightly better protection. Overall, full effectiveness for most current COVID-19 vaccines is generally reached within two weeks after completion of the primary vaccine series or booster dose.