Preliminary data reveals that the standard doses of COVID-19 vaccines are not very effective against infection from the surging omicron variant. But the new research also found that, especially with mRNA vaccines, booster shots improve protection.

According to Axios, not everybody is eligible for this crucial booster shot. Anyone aged 16 and older who got their Pfizer or Moderna vaccine at least six months ago can get the booster, along with those who had the Johnson & Johnson shot at least two months ago.

That leaves out about 58 million Americans who received their second mRNA shots within the last two months, says the Centers for Disease Control and Prevention. Children from 5 to 11 years of age just became eligible for vaccines last month so this population also isn’t eligible for a booster.

Overseas, countries are changing gears about the timing of booster shots. In the U.K., adults are eligible for booster shots three months are receiving their second dose of vaccine. In Denmark, officials announced that people over the age of 40 can get their third shot 4.5 months after the second shot.

Some experts disagree with moving up the timing of the third shot, arguing that longer intervals between vaccines seems to induce better antibody responses.

“Three doses in a short interval might not provide substantial benefit,” said Angela Rasmussen, a noted virologist, according to Axios. The good news is that a South African analysis found that two Pfizer shots are 70% effective against hospitalization, so while you have a greater chance of infection from the omicron variant, your chances of severe illness are reduced by two doses of the vaccine.

Timing is everything when it comes to boosters, says Yahoo! News. In fact, the longer you wait, the more effective the booster may be in enhancing your immune system. Israeli data found that the Pfizer vaccine began to lose its efficacy after six months. “The immune system generally does better in terms of ‘remembering’ if you wait a few months before the last shot,” said Dr. Peter Chin-Hong, an infectious disease physician at the University of California, San Francisco, referring the booster as the third shot.

Dr. Paul Offit, the director of the Vaccine Education Center at Children’s Hospital in Philadelphia says that rushing into shorter intervals for boosting should be made with clear data in mind. People vaccinated less than six months ago “should be assured that they are in all likelihood protected against serious illness,” he explained.