It’s not known exactly how long vaccination lasts, particularly against the omicron variant. But studies of previous variants have suggested that COVID-19 vaccination protects against infection or mild disease for several months, while protection against more serious illness is longer-lasting, on the order of six months or more.
A systematic review of studies conducted prior to omicron, for example, found that for the four main COVID-19 vaccines used worldwide, protection against severe disease for those fully vaccinated fell by about 10 percentage points over time but remained above 70% at six months. In contrast, protection against infection or symptomatic disease fell more rapidly, falling by 20 to 30 percentage points by six months. Some declines were more pronounced in older people.
Because of waning immunity and the ability of some coronavirus variants to also evade immunity, the Centers for Disease Control and Prevention recommends most people get a booster dose, which can restore some of the protection. People over the age of 50 and people who are immunocompromised are also eligible for a second mRNA booster.