CDC’s definition of ‘fully vaccinated’ has not kept up with the science

A year ago, the Centers for Disease Control and Prevention (CDC) defined “fully vaccinated” against SARS-CoV-2 as having received one dose of the Johnson & Johnson (J&J) vaccine or two serial doses of the Pfizer or Moderna vaccines. This definition has been used extensively across the country to establish requirements for public engagement, from work to schools to restaurants to Broadway shows, under the interpretation that those who are “fully vaccinated” are those who are safest from acquiring and spreading the virus.

Fortunately, we’ve learned a lot of science in the last year about vaccine durability, protection from new variants, as well as natural immunity. We also have much more granular ways to evaluate the likelihood that someone will put others around them at risk of COVID-19. Unfortunately, CDC’s definition has not kept up with the science, and the year-old national standard has not aged well.

We learned months ago that vaccines have limited durability, particularly in their ability to protect us from acquiring, replicating, shedding and then spreading the virus. Five months after being “fully vaccinated,” we might still have a decent T-cell repertoire to reduce the chances of getting very sick from COVID-19. However, our antibody levels have likely dropped substantially, and with them, our first line of defense against acquiring and replicating — and thus shedding and spreading — the virus. This is particularly relevant with the new omicron variant, which is so rapid and contagious that we need even higher levels of antibodies than we ever have before.

Boosters can help rekindle antibody production and immediate early protection, and CDC is wisely recommending boosters. But inexplicably, they are not incorporating boosters in the definition of “fully vaccinated.” Someone vaccinated a year ago has a much higher likelihood of putting others around them at risk of COVID-19 than someone boosted a month ago, yet CDC puts both in the same “fully vaccinated” category.

The protection afforded by boosters is on the order of 20-fold higher than without boosters, about the same magnitude as the protection afforded by vaccines in the first place. So, if it were important to distinguish between vaccinated and unvaccinated when CDC first established this definition, now it is similarly important to distinguish between boosted and unboosted. To put it more bluntly, someone whose last dose of the SARS-CoV-2 vaccine was over five months ago should no longer be considered “fully vaccinated” and is likely no longer protected enough to be around strangers indoors. Sure, they might be fine if they get COVID-19, but their chances of infecting others in that room have likely become unacceptably high.

Furthermore, the entire concept of natural immunity has been ignored by definitions that focus on “fully vaccinated” rather than “appropriately immune.” Indeed, even for those with previous COVID-19 infection, it remains essential to get vaccinated and boosted, as hybrid immunity seems to be the most powerful weapon against SARS-CoV-2. However, today it seems hard to reconcile that an unvaccinated person who recovered from moderate COVID-19 last month, and probably has very high antibody levels, is considered less “safe” than someone vaccinated a year ago, who probably has very low antibody levels.

Several things have changed over the last few months to make addressing the role of natural immunity an imperative. First, we now have a much better sense of how to measure and interpret antibody levels. Second, vaccine immunity is waning, so the chosen standard to which natural immunity has been held is now widely heterogeneous. Third, thanks to the omicron variant, there are now tens of millions of Americans who had very recent infections and, thus, likely high immunity right now.

CDC needs to expand its definition for two groups contending with natural immunity. First, there are many Americans who were hesitant about vaccines, got COVID-19, and remain hesitant, holding their ground that their natural immunity is at least equivalent to vaccination. Demonstrating a certain level of antibodies could be a way for these folks to support this stipulation. Antibodies are now well-established to correlate with neutralization and clinical protection. While not perfect, and a bit challenging to identify acceptable antibody tests and protective levels, it would certainly be better than the current CDC definition that ignores natural immunity altogether. CDC could also use an estimate of natural infection durability, similar to that used for vaccine durability, possibly requiring antibody demonstrations only from those outside this durability window. Such a paradigm could also be used for those who were vaccinated but are booster-hesitant, or those who wish to wait longer before seeking a booster: demonstrate that your antibodies are high enough, and you buy yourself more time. 

Second, there are many people — especially since omicron surged — who were initially vaccinated, soon to be eligible for boosters, but got COVID-19 before getting boosted. CDC needs to clarify guidance for when to get boosted and update the definition of “fully vaccinated” to account for this natural booster. As above, this could be a combination of an estimated durability window and antibody demonstration for those outside the window.

Like it or not, the CDC definition of “fully vaccinated” has become the de facto rule of public engagement — an indication of how safe someone is to be around strangers, adopted by many institutions across the country — and not a biological indication of how sick someone will become if infected. Sadly, right now, this definition is embarrassingly outdated, ignoring new science about vaccine durability, new variants and natural immunity. This is causing major problems, both medical and political, as well as intense confusion. It’s been a year. It’s time for an update.

Dorry Segev, MD, Ph.D., is a professor of surgery at Johns Hopkins University School of Medicine and Professor of Epidemiology at Johns Hopkins Bloomberg School of Public Health. Segev has been leading an observational study of COVID-19 vaccine responses in immunosuppressed people since December 2020 and is the principal Investigator of the NIH/NIAID-funded interventional trial “COVID-19 Protection After Transplantation (CPAT).” Follow him on Twitter: @dorry_segev