Calling the eradication of the COVID-19 virus “unlikely,” a UK scientific advisory group says (PDF) there is a “realistic possibility” that a variant will emerge that is resistant to the current battery of vaccines. Governments, public health organizations and vaccine makers are all tracking developments in coronavirus variants like delta and lambda, hoping to answer the question if booster shotstargeting new variants will be needed soon.

“Things are going to get worse,” Dr. Anthony Fauci, the chief medical advisor to the president, said Sunday on ABC’s This Week.

Currently in the US, “breakthrough” coronavirus cases caused by the dominant delta variant amount to less than 1% of people who are fully vaccinated. Both Moderna and Pfizer vaccines are proven to be over 90% effective against hospitalizations and death. The surge in new COVID-19 cases are primarily affecting unvaccinated people and causing community spread, and in turn, the return of mask mandates and guidance in hard-hit areas, even for people who have full vaccine protection. The debate over mask use and vaccine boosters underscores how scientists and other health experts continue to grapple with the uncertainties of COVID-19.

The Centers for Disease Control and Prevention’s recent study shows that vaccinated people can both contract the highly contagious delta variant and spread it. According to a widely reported internal CDC memo, the delta variant spreads as easily as chicken pox, which is considered more contagious than the flu and less contagious than measles.

To prepare for the possibility of a booster shot, the CDC said it’s weighing a third vaccine dose for people with compromised immune systems. Over the weekend, Israel began administering third doses of the vaccine to those 60 and older, and the UK plans to do the same. However, this is resulting in a backlash among countries that are struggling to deliver first and second shots to residents.

On Wednesday, World Health Organization Director-General Tedros Adhanom Ghebreyesus called for a “moratorium” on booster shots in high-income countries, citing the global disparity in vaccine distribution. Of the 4 billion doses administered globally, 80% have gone to high- and upper-middle income countries that make up less than half of the world’s population, he said.

“We cannot accept countries that have already used most of the global supply of vaccines using even more of it, while the world’s most vulnerable people remain unprotected,” Tedros said.

What does all of this mean in the US? Here’s what we know about COVID booster shots now.

Booster shot vs. new COVID vaccine: There’s a difference

Along with Moderna, Pfizer’s current two-dose vaccine provides powerful and effective protection against all known variants of COVID-19, including the delta variant, according to ongoing studies and self-reported statistics. But Pfizer also announced in July that a third dose of its vaccine is currently under development. The company said its own research showed a booster shot of its current vaccine increased antibody levels five to 10 times higher over its two-dose shots, noting that its results have not been published or peer-reviewed.

Pfizer said it believes the level of protection the first two doses of its vaccine provide can gradually decrease over time, and a third booster dose may be needed “within six to 12 months” after a person is fully vaccinated with the first two doses. Pfizer said a booster shot could enhance protection against the delta variant, which has been known to infect fully vaccinated people. Clinical trials on the booster are set to begin, as Pfizer seeks approval from government regulators for a third dose.

However, while a booster shot would complement the two doses of its existing vaccine, Pfizer is also separately working on a new vaccine formulation targeting the delta variant.

What do the CDC, FDA and WHO think about a booster shot?

The CDC and FDA initially exercised caution over a booster shot.

“People who are fully vaccinated are protected from severe disease and death, including from the variants currently circulating in the country such as delta,” the CDC and FDA said in a July 8 joint statement, without naming Pfizer. The government agencies emphasized the need for all eligible people to receive full doses of one of the approved vaccines, all of which are free.

The CDC and FDA said the question of a booster requires extensive scientific data and doesn’t depend on the input from pharmaceutical companies alone. “Virtually all COVID-19 hospitalizations and deaths are among those who are unvaccinated,” the statement mentioned, adding that the agencies will approve booster doses “if and when the science demonstrates that they are needed.”

New research has demonstrated a need. There is a growing consensus among Biden administration health officials that older individuals and those with compromised immune systems may, in fact, need a third shot. At a White House press briefing Thursday, Fauci said that it is “extremely important for us to move” to get immunocompromised people a booster, citing observational data that they generally aren’t “adequately protected” with the standard vaccine dose.

“We are now working on that, and we’ll make that be implemented as quickly as possible,” Fauci said. About 2.7% of the US adult population is immunosuppressed, CNBC reported, but immunocompromised people make up about 44% of people hospitalized with COVID complications.

Earlier in the week, at a WHO press briefing Wednesday, Tedros said that while he understands “the concern of all governments to protect their people from the delta variant,” booster shots shouldn’t be prioritized until the world’s most vulnerable people and health care workers are vaccinated.

“We call on vaccine producers to prioritize Covax,” Tedros said, referring to the world’s COVID-19 vaccine distribution program.

Is Moderna also planning to develop a booster shot?

While scientists and public health officials continue to study if those who are fully vaccinated will need a booster shot, Moderna said — along with Pfizer — it is exploring a third vaccine dose to complement the initial two vaccine shots.

I got Johnson & Johnson — Do I need a booster?

No, fully vaccinated Americans don’t need a booster yet, including those who got the single dose of Johnson & Johnson’s COVID vaccine, according to the country’s health officials. But residents in San Francisco who received Johnson & Johnson’s one-dose COVID vaccine were given the green light Tuesday to get a dose of an mRNA vaccine, though it still isn’t recommended by the city’s health department.

Dr. Grant Colfax, San Francisco’s health director, said that there isn’t conclusive evidence that getting a dose of Pfizer or Moderna benefits those who got the J&J shot, but there’s also not evidence to show it’s harmful, the San Francisco Chronicle reported.

“If people received the Johnson & Johnson and are requesting a second shot, we will accommodate them, but our policy has not changed,” Colfax said.

San Francisco’s decision to legitimize Johnson & Johnson vaccine recipients getting an mRNA vaccine comes in light of a small study that suggests the vaccine isn’t nearly as effective against the delta variant as the other vaccines. (Another study suggests that Johnson & Johnson remains effective, and the drug-maker continues to claim the vaccine is effective.)

Would the booster shot be free?

The current one-dose vaccine shot from Johnson & Johnson and two-dose versions from Moderna and Pfizer are free to anyone who wants to get vaccinated. According to the Biden administration, COVID-19 booster shots will also be free, if and when they’re approved.

Is it a good idea to mix and match COVID vaccines?

The CDC doesn’t recommend mixing and matching vaccines from the different makers, saying it hasn’t evaluated the effectiveness of mixing vaccine doses and that the “vaccines are not interchangeable.”

However, other global health agencies and countries are testing administered vaccines from two different manufacturers. In the UK, for example, a recent study found that those who received a first dose of the AstraZeneca vaccine and a second of Pfizer had a higher immune response than those who received two doses of the AstraZeneca vaccine.

In early June, the National Institutes of Health started a trial that tests mixing COVID-19 vaccines, and will determine if those who got Pfizer or J&J initially can safely receive a Moderna booster. The initial results of the study are expected in late summer 2021, the NIH said in a news release.

While we wait to see how the situation develops, here’s what we know about the delta variant, more about COVID-19 boosters and if you need to continue to wear a mask.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.