ROCHESTER, Minn. As health officials prepare to expand eligibility for COVID-19 boosters to all adults, a surprising picture is emerging of who that step would benefit the most.

Thus far, early evidence for universal boosters has been largely derived from a controlled trial by one manufacturer and observational data from Israel, the UK and the American VA system.

Though trials of the original two-dose series showed high rates of protection from infection, some combination of the passage of time and the high transmissibility of delta variant appears to have reduced that protectiveness.

The reduction however, is one from heights that were once soaring, to those that are only modest. The good news is that boosters appear to return that protection from infection to its original heights — at least for the short term.

Many of these findings, moreover, are unique to Pfizer. The Moderna vaccine created double the Pfizer antibody response, and therefore did not drop as precipitously, while the one-dose J&J vaccine was determined last summer to be incomplete without a second shot.

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To grateful booster recipients now sharing "boosted" messages on social media, third shots are widely envisioned as a strengthened firewall against infection and serious illness. For those who are older or have complex health conditions that appears to be the case.

For all others, however, experts increasingly depict boosters in terms of population health, as opposed to personal health.

In short, though they reduce the risk of infection for the healthy the primary value of a third shot may have less to do with protecting the fully vaccinated, than limiting the unknown risk they add to the considerable danger faced by the unvaccinated.

"I tell young, healthy persons the booster is more likely to help with decreasing the overall burden of infection in the community," said Mayo Clinic infectious disease specialist Dr. Abinash Virk during a media call recently.

"It may not be so much in terms of protection for them from getting it, or going to the hospital and dying from it, but more for us to decrease the overall burden of infection in our country as a whole."

Dr. Abinash Virk via screenshot from Mayo Clinic News Network
Dr. Abinash Virk via screenshot from Mayo Clinic News Network

Breakthrough rates 'extremely low'

Health officials stress that vaccination and masking indoors are the primary tools for slowing spread.

The unvaccinated have 13-15 times higher rate of infection than the vaccinated, Virk said, while those with only natural immunity have 5 times the rate of reinfection than the infected who subsequently got vaccinated.

If boosters have been embraced as a life raft to the vaccinated, Virk, who supports them as advised by state health officials, describes breakthrough rates as "still extremely low," affecting just 1.8% of people who were vaccinated.

Vaccination is keeping people out of the hospital she adds.

"The (initial) protection against hospitalization, death and severe disease has stayed extremely stable," Virk said, "despite the effects of delta and waning immunity."

This is the backdrop to the complex case now being made for universal boosters, including experts now weighing in to support third doses for all adults six months after their second shot.

"The first two doses continue to provide high degree of protection against severe disease," argued Mayo Clinic hematologist Dr. Vincent Rajkumar on Twitter recently.

"But because even mild breakthroughs can spread infection to vulnerable and unvaccinated populations, we have to try to get all cases down."

"Let’s consider a healthy, 30-year-old vaccinated woman," as Dr. Ashish. K. Jha, dean of Brown University School of Public Health wrote Thursday, Nov. 18, in The Atlantic.

"It’s true that the benefits of a booster for her individually are modest: With waning immunity, she’ll be at risk of a breakthrough infection when she encounters the Delta variant, but for her that would likely mean a few days of misery and full recovery, with a small (albeit not zero) risk of severe disease or long-term complications."

But as population-health, Jha says, boosters are hard to dismiss.

"That healthy 30-year-old might want to visit older relatives or be around an immunocompromised friend. While her risk of spreading the virus if infected is low, it would be much lower if she were boosted."

There also are dosing arguments. Rajkumar points to data showing booster effectiveness at preventing infection, but also the belief that "the first two doses were spaced too close, and a booster is needed for all adults," to fulfill the vaccine's untapped initial potential.

Recent studies support this view, Virk says, showing the antibody response of a third shot far surpasses that of the second shot.

After the broad array of those recently approved, all this careful reasoning may ultimately affect only a small number of new booster recipients.

Third shots were already available to many Americans, groups chosen on differing criteria, some more widely embraced than others.

For the immunocompromised, who do not develop a robust immune response to the standard dose of vaccine, a third shot would not even be considered a booster, Virk says, but a baseline inoculation with a booster at six months following shot number three.

Boosters have also been recommended for persons who are 65 and older and those with chronic conditions, groups for whom age or poor health may have caused them to produce an insufficient immune response to the second shot.

Boosters have also been allowed for those in high-risk populations and living in long-term care, groups granted use of the shots against the advice of an expert panel when CDC Director Dr. Rachel Wolensky overruled their recommendation.

Now those distinctions have dissolved in a blanket expansion of eligibility to all adults.

It comes at a time of high strain on the health system.

Virk says the Mayo Clinic Hospital in Rochester ICU and progressive care facilities are currently between 70-94% capacity, while the health system is between 80-100% capacity, with 14% of acute care and ICU space in most of clinic facilities now treating COVID-19.