Why Vaccinated People Can Still Get COVID-19
Breakthrough infections occur when people who have received a COVID-19 vaccine become infected with the virus. These infections are possible because immunity is not a perfect barrier against every exposure.
One important reason is that immune protection can change over time. Antibody levels may decline after vaccination or infection, and the virus can evolve in ways that help it partially evade existing immune defenses. A variant that differs from the virus targeted by an earlier vaccine may therefore cause infections even among people who have some immunity.
Exposure also matters. Someone who lives with an infected person, works in a crowded environment, or frequently encounters the virus may have more opportunities to become infected than someone with limited exposure.
In addition, people differ in how strongly they respond to vaccination. Older adults and people with certain medical conditions or weakened immune systems may develop a less robust response than younger, healthier individuals.
A breakthrough infection does not automatically mean that vaccination provided no benefit. Vaccines are designed to reduce the risk of serious disease, and the degree of protection can differ depending on the outcome being measured. Preventing every infection is more difficult than reducing the chance that an infection will lead to hospitalization or death.
Vaccinated people should therefore continue to pay attention to symptoms and take sensible precautions when illness is circulating, particularly if they or someone close to them is at increased risk of complications.
Protection Against Severe Illness and Hospitalization
One of the most important benefits of COVID-19 vaccination is its protection against severe outcomes. Vaccination has been associated with reduced risks of hospitalization and death, although the size and duration of the benefit vary across populations, vaccine formulations, variants, and time periods.
The distinction between infection and severe disease is important. A vaccinated person may test positive and experience symptoms, yet still have a lower risk of developing serious complications than they would have without the protection provided by vaccination.