With the emergence of new COVID variants, immunity from vaccines and that from natural infection seems to have diminished considerably. The heavily mutated Omicron is said to be milder than the previous variants, however, it infected many in just a short span of time, irrespective of whether the person is vaccinated or has had COVID.
As far as vaccine-induced immunity is concerned, based on emerging data, the World Health Organization (WHO) says that there has been a consistent decline in vaccine effectiveness against SARS-CoV2 infection and COVID-19 with time since vaccination, and more significant decline in older adults.
In light of diminishing immunity and prevalence of breakthrough infections, the demand for vaccine boosters has increased tremendously. Booster shots help re-expose the immune system to the immunizing antigen, the memory of which may have waned over time.
In India, booster vaccines, known as precaution dose, are being given to fully vaccinated healthcare and frontline workers and senior citizens aged 60 and above.