LOS ANGELES -- There's no simple formula to guide Americans' decisions about booster shots intended to increase protection against severe covid-19 illness, according to experts.
Whether to get a booster shot and which one to get depends on who the person is, what medical vulnerabilities the person has and what vaccine the person got first, authorities say. Who the person lives with and the kind of work the person does might also influence the decision.
And then there's the deeply personal matter of how much risk -- of covid-19 or of vaccine side effects -- the person is willing to accept.
Even if a vaccine's protection has slipped with time, many fully vaccinated young people, or those who've had covid-19 infections before or after being vaccinated, can reasonably decide that their likelihood of becoming very sick with the virus remains low, experts say.
The experts who advised the Centers for Disease Control and Prevention last week made clear that they were not recommending boosters for all. Rather, they were recommending that millions of people who are fully vaccinated have access to a booster shot if they want it.
A new government study on mixing and matching vaccines has added more options -- and thus more complexity -- to the issue of boosters.
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The results so far are preliminary, and the participants are still being tracked. But after at least 12 weeks of follow-up, researchers haven't detected any ill effects in people who were first inoculated with one covid-19 vaccine and then got a booster shot of another.
The reassuring findings led Dr. Rochelle Walensky, the CDC director, to leave the choice in patients' hands.
"Some people may have a preference for the vaccine type that they originally received and others may prefer to get a different booster," the agency said in new guidance issued late last week. "CDC's recommendations now allow for this type of mix and match dosing for booster shots."
Now, in addition to a person asking whether he needs a booster, there's the question of whether he should seek out a shot that's different from the one he got the first time around.
MAXIMIZING THE BOOST
If a person is looking to maximize his immunity boost, he's out of luck. The data are not yet in to guide inform that decision.
The new study includes three groups of 150 fully immunized people -- one who got the Pfizer-BioNTech vaccine, one who got the Moderna vaccine, and one who got the Johnson & Johnson shot.
Researchers took each group and further divided it into three subgroups, with 50 people in each. Participants in one arm of each subgroup got boosters with the same vaccine they originally received.
Each group's second and third arms got boosters with one or the other of the remaining vaccines authorized for use in the United States.
At this point, not enough people have been tracked for a long enough period for scientists to make meaningful comparisons among groups.
The study's early efforts to measure jumps in coronavirus-fighting antibodies showed that mixing was generally no worse than matching, and that it was sometimes better.
With regard to the Johnson & Johnson vaccine in particular, the data suggest that those who got an initial Johnson & Johnson shot had their antibody levels rise a bit more after 15 days if they received boosters of the Pfizer-BioNTech or Moderna vaccines than if they got second doses of Johnson & Johnson.
When either the Pfizer or Moderna boosters were used after any of the three primary courses of vaccine, the boost in antibodies was about the same.
Safety considerations, then, offer the best guide for who should consider mixing and matching.
THE CONSIDERATIONS
Through Oct. 13, vaccine safety monitors detected 47 cases of a rare blood-clotting disorder in people who received the single-dose Johnson & Johnson vaccine.
That's a tiny fraction of 15.3 million doses of Johnson & Johnson vaccine administered. But the disorder -- called thrombosis with thrombocytopenia syndrome, or TTS -- is life-threatening. Five people have died of the condition, which causes runaway clotting that is difficult to treat.
Three-quarters of Johnson & Johnson vaccine recipients who developed TTS were women ages 18-49. Four of the five who died were women.
Tests that might predict a person's risk of developing TTS aren't readily available. So women under 50 who want more protection might consider seeking out the Moderna or Pfizer-BioNTech vaccine for their booster shots, experts say.
For every million young men ages 18-24 who got second doses of one of the mRNA vaccines, safety monitors detected somewhere between 37 and 39 cases of myocarditis or pericarditis -- essentially an inflammation of the heart muscle or its surrounding tissue.
The mRNA vaccines teach cells how to make a protein that triggers an immune response. Pfizer-BioNTech and Moderna are mRNA vaccines.
The rate of myocarditis or pericarditis was highest among men under 21, but it was seen with decreasing frequency in men up to 29. It was also seen in women, though far less frequently. And it was slightly higher in young men who were initially inoculated with the Moderna vaccine than in those who got Pfizer-BioNTech.
The conditions were much more common after the second dose than the first. Some experts fear a third dose could further escalate the risk.
Myocarditis and pericarditis are rare side effects, and in more than three-quarters of cases, the symptoms -- tightness, squeezing or pain in the chest -- went away with time and over-the-counter drugs. However, no one knows if a bout of inflammation will have long-term effects.
To be safe, cardiologists generally recommend cessation of strenuous exercise for three to six months after a case of either condition. If a young male athlete who'd gotten one of the mRNA vaccines felt he needed a third shot, he might consider getting the Johnson & Johnson booster, or maybe switching from Moderna to Pfizer-BioNTech.
ANOTHER CONSIDERATION
Guillain-Barre syndrome is a reaction to infection or vaccine in which the immune system attacks the nerves, sometimes causing paralysis that can last months.
The Food and Drug Administration's adverse event monitoring system has picked up at least 130 reports of people across the age spectrum developing the syndrome after getting the Johnson & Johnson vaccine, but no rise among those who got mRNA vaccines.
Last week, the British government added a warning about rare cases of the syndrome following use of the covid-19 vaccine made by AstraZeneca, which is not authorized for use in the U.S. but is similar in design to the Johnson & Johnson vaccine.
Anyone with a personal or family history of Guillain-Barre syndrome who got the Johnson & Johnson vaccine should consider switching to one of the mRNA vaccines as a booster, and should probably avoid the Johnson & Johnson vaccine as a booster to an initial course of Pfizer or Moderna, experts say.