ANALYSIS- Here's why there may be no need for boosters for everyone just yet

Last week, the US Centers for Disease Control and Prevention and the US Food and Drug Administration authorized and recommended giving a third dose of vaccine to people with compromised immune systems -- not as a booster, but because their immune systems are unlikely to have responded properly to the first two doses of Moderna's of Pfizer's vaccines.

States, cities and hospital systems have already begun offering these doses.

Then, Monday night, administration sources shocked the medical community by saying federal officials were leaning toward offering booster doses more generally to people.

The FDA, the agency that would make the actual decision, declined to comment directly. "FDA, CDC, and NIH are engaged in a science-based, rigorous process to consider when boosters might be necessary," the FDA told CNN in a statement Tuesday.

While Pfizer, especially has been pushing for booster doses, and said Monday it had submitted data to FDA to support the idea, the FDA statement pushed back.

"This process takes into account laboratory data, clinical trial data, and cohort data -- which can include data from specific pharmaceutical companies, but does not rely on those data exclusively. We continue to review any new data as it becomes available and will keep the public informed," it said.

The CDC also pushed back, saying while some immune compromised people need a third dose, there's not a clear need for boosters.

"Other fully vaccinated individuals do not need an additional dose right now," the CDC's Dr. Kathleen Dooling, who leads the agency's Advisory Committee on Immunization Practices, told doctors and other medical providers on a call Tuesday.

The FDA and CDC have been saying there is no clear argument yet for giving boosters to people with normal immune systems. While there are fresh surges of virus across the country, nearly all of the people becoming severely ill with Covid-19 now are completely unvaccinated.

Scott Hensley, an immunologist at the University of Pennsylvania, said he was surprised at reports boosters were being considered for the general public.

"If you look at hospitalizations and deaths in the United States right now, they are almost exclusively in unvaccinated individuals," Hensley told CNN.

"If you want to reduce global deaths and hospitalizations, well, the answer is simple. You have got to get the vaccine people that haven't been vaccinated before. I am afraid a rollout of boosters in United States will take away efforts from getting the vaccine to the developing world."

The World Health Organization agrees, and has been saying so in ever-stronger terms.

"What our recommendation is, is that all of the world's most vulnerable, and those who are most at risk, health workers, need to receive their first and second doses before large proportions of the population, or all of the population in some countries, receive that third dose," Maria van Kerkhove, WHO's technical lead for Covid-19, told CNN Tuesday.

"If you think a third dose of the vaccine is going to end the pandemic, then you are kidding yourself."

"Of course we want people to be protected and to receive the full course. But what we are trying to move against is giving a third dose to people who already are well protected."

Plus, plenty of Americans -- around 90 million eligible people—remain unvaccinated. They're the main source of spread, Hensley noted.

"If you think a third dose of the vaccine is going to end the pandemic, then you are kidding yourself," he said.

"The way to end this pandemic is to get the vaccine distributed across the globe."

Virologist Andy Pekosz of the Johns Hopkins Bloomberg School of Public Health agrees.

"Boosters don't preclude the fact that we need to get out there and get people vaccinated for the first time," Pekosz told CNN.

"We need to get people who've been infected or feel they don't need to be vaccinated, we need to get them to get vaccinated because it's targeting those populations that is really going to help us control this pandemic," he added.

Pekosz said it might make sense to offer booster doses to people most at risk of severe disease, such as the immune compromised or the very elderly, but not to the general population just yet.

"Booster shots are going to help us with severe disease. We control a pandemic by getting more of the population vaccinated," he said.

Pfizer's argument for a booster rests on research that shows levels of immune system proteins called antibodies shoot up after people get a third dose. That's true, Hensley says.

But the human immune system has other components, too, and it may not be the antibodies that are the most important when it comes to long-term immunity.

"Antibodies are sort of our first line of defense," Hensley said. "If we have high levels, they can recognize the fires. That's what prevents the virus from even getting into our cells in the first place."

The Delta variant of the virus appears to evade these antibodies to some degree. And antibody levels do wane over time -- even though studies show they stay at protective levels for six to eight months and possible longer after people get all three US authorized vaccine -- those made by Moderna, Pfizer/BioNTech and Johnson & Johnson.

Once those antibodies start to wane, the body relies on T cells, Hensley said.

"T cells can't prevent infection. That's not how they work. But what they do is they get rapidly recruited and they clean up the infection, and they keep you out of the hospital and they keep you from dying," he said.

That's why a third dose might not be needed for most people. Their T cells will protect them.

"The third vaccine dose, what it will probably do is it will boost antibody responses to a level that is very high and those antibodies will be at a higher level both against the original strain, as well as the Delta strain," Hensley said.

"Even if you are the most selfish person in the world, and you only care about yourself, you should still stand behind this idea of getting the vaccine distributed across the world."

That looks good on paper, but in terms of real-life protection, it may not add much.

"The third dose will likely do very little for further boosting the vaccine's ability to protect against severe infections and deaths. And that's because it's already pretty darn good. If you're already 90-95% effective at preventing deaths, which the standard two dose regimen is, well, it's hard to get much better than that," Hensley noted.

This is why it is more important to simply vaccinate more people. If more people are vaccinated, they will be breathing less virus into the air -- and the potential for breakthrough infections falls.

So does the risk of new variants that may eventually develop the ability to completely evade vaccine protection.

"The only way that we won't have new variants is if we can limit the spread of the virus," Hensley said.

"Even if you are the most selfish person in the world, and you only care about yourself, you should still stand behind this idea of getting the vaccine distributed across the world, because it's the only way that you're going to limit new variants from emerging."

Pekosz ageed.

"We need to think about this as a global disease, and make sure that the vaccines get out there to control virus replication anywhere, because a new variant can emerge anywhere. And that variant can make its way around the globe very, very quickly," he said.

"We've seen that over and over again in just this short term of the pandemic."

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