measured. Protection against mild or symptomatic infection may decline differently from protection against hospitalization and death. This is one reason health authorities evaluate vaccine effectiveness using several endpoints instead of treating all infections as equivalent.
The WHO notes that immunity can decline relatively quickly within months after vaccination or infection, although the degree and consequences of that decline vary. Its current guidance emphasizes vaccination for groups at the highest risk of severe disease, with national recommendations determining the appropriate schedule.
2. The virus continues to evolve
Viruses change over time.
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