The FDA authorized distribution of new formulations of the COVID-19 mRNA vaccines from Moderna and Pfizer/BioNTech on August 31st based on safety data in humans and efficacy data from animal studies. Both vaccines have two mRNA components, one encoding the spike protein from the original SARS-CoV-2 strain and a second encoding the different spike protein from the current variants (BA.4 and BA.5). Bivalent simply means that the vaccines have two components. The Moderna vaccine contains 25 micrograms of each mRNA and is authorized for individuals 18 years and older. The Pfizer/BioNTech vaccine contains 15 micrograms of each mRNA and is authorized for children 12 year or older as well as adults. Some doctors recommend staying with the same vaccine manufacturer that you received previously, although there is no obvious harm in switching that has emerged to date.
The bivalent booster shots are recommended if 6 months have elapsed since your last booster shot with original Moderna or Pfizer/BioNTech vaccine formulation or primary immunization with the Johnson and Johnson vaccine. If you have recently had a COVID-19 infection (probably from the more prevalent BA.5 variant), you are advised by the CDC to wait 3 months before getting the bivalent booster shot.
The immune response in older individuals is less robust and wanes earlier than that seen in younger adults. The CDC recommendation of waiting 6 months since your last booster is really focused on the general population, and those over 65 may benefit from getting the bivalent booster as soon as 3-4 months after their previous booster.
We don’t know how effective the bivalent boosters will be in preventing infection, hospitalization, or death due to COVID infection. These data are being gathered as the new vaccines are being administered, so results will become clear over the next few months. One major concern is that a new variant may emerge this winter that is sufficiently different from the BA.5 variant that it evades the immune response triggered by the new vaccines. While 68% of the U.S. population is fully vaccinated (as of mid-September), that still leaves a large fraction of unvaccinated individuals who are highly susceptible to infection and who will not get the bivalent vaccines.
Deaths from COVID infection in the U.S. have plateaued at around 400 per day for the last two months. Deaths are more common in older individuals with underlying conditions, but this death rate underscores how dangerous this disease is for vulnerable individuals. Moreover, it makes it clear that we have not controlled this pandemic and the virus is here to stay.