The CDC has recommended that adults over 50 years of age receive a second booster shot of an mRNA vaccine (Pfizer or Moderna) if 4 months have elapsed since their first booster. This advice was based on two Israeli studies in individuals 60 years or older that showed that a second booster shot reduced serious illness by almost 80% compared to a control group that had received only the first booster. Protection against infection was much less impressive and waned within 4 weeks of the second booster.
Vaccine protection against infection is difficult to achieve in any setting, but vaccine protection against serious disease is an expected result, meaning that the immune system has been primed to recognize the virus and to eliminate it before it can cause severe symptoms. The CDC recommendation for a second booster for older adults and immunocompromised individuals recognizes the reality that the immune system gets weaker as we get older.
The BA.2 variant of the COVID-19 Omicron strain is currently spreading across the United States and Europe, although local cases have not yet increased substantially as of this writing. A second booster should help protect against serious illness caused by BA.2 infection, although limited data support this conclusion. The second booster will not protect against infection with BA.2, at least for not very long, so wearing masks when visiting indoor settings is still a valuable protective strategy. In addition, rapid COVID antigen tests are widely available now, and you can check your infection status before or after any large event that might result in exposure.
COVID-19 and its current and future variants are here to stay. Vaccine resistance and the emergence of highly transmissible variants mean that we are likely to experience waves of infection not unlike seasonal flu outbreaks. An emerging scenario is that we will need to get an annual (or perhaps more frequent) booster of a modified vaccine targeting the current variant to stay protected against serious illness.