Boosters and Mixing Shots

There has been extensive media attention over the past week devoted to mixing of COVID-19 vaccines and booster injections. As expected, those individuals in test trials that received a mix of vaccine types or additional shots have shown no adverse events. The vaccines are safe and interchangeable. Attention to these studies has prompted many to ask whether mixing of vaccine or a booster is beneficial to them, and the answer is no, not yet, in most circumstances.

What we read in the media is often from small studies done in laboratories and not based on true experience. Conclusions from these studies are drawn from measuring antibody levels in plasma rather than demonstrated immunity in a person. The studies are definitely important to help understand the virus, and they are sometimes newsworthy. However, the results from small lab studies often conflict. For example, limited data from waning plasma levels in vaccinated individuals suggest that boosters will be needed annually. Other studies show that that immunity from T cell responses may offer protection from the virus for years.

Thus, these preliminary studies should not to be used to guide medical practice, except in limited circumstances, and are meant to aid in the design of larger, more definitive studies. The correlation between high plasma antibodies and degree of protection from a given pathogen is not always reliable. Changing vaccine recommendations without substantial supporting data is irresponsible, expensive, and introduces complexity that ultimately may lead to unintended consequences.

The best way to determine when boosters or mixing of vaccine will be needed is by performing prospective trials of those who originally received vaccine. These studies are underway. We will know it is time for boosters when the number of breakthrough infections exceeds what would otherwise be expected. Our current vaccines and our current processes are working extremely well, even in the face of variants. This is particularly true in areas with high vaccination rates and low virus prevalence.

Our focus needs to remain on vaccinating those who have been waiting, on the fence as to when to get the shot. Analysis by Johns Hopkins and others released this week show a dramatic difference in hospitalization rates between counties and states with low vaccination rates compared to those with high vaccination rates.

Pfizer and BioNTech made public this week their plan to seek FDA authorization for a third dose of their COVID-19 vaccine in anticipation of future needs at some point in the near future. The booster will likely be modified to target delta and other variants. Such an announcement is welcoming while also instilling unintended anxiety in many of us, raising the question of timing. Remember that a booster is not authorized at present, and consider that the current vaccine you have received remains highly effective at preventing disease and limiting transmission. Our risk as vaccinated individuals in an area with high vaccination rates is very low.

We will continue monitoring the characteristics of the virus as it evolves. Locally and at the state level, we have the resources and preparation to provide boosters and react accordingly when appropriate. 

Trey Dobson, MD, is an emergency physician at Dartmouth-Hitchcock Medical Center and the chief medical officer at Southwestern Vermont Medical Center in Bennington.