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(WGHP) — As states expand eligibility for COVID-19 vaccinations to more and more people, some are beginning to wonder why the age requirement isn’t consistent across the three currently approved vaccines.

To get the Pfizer vaccine, you must be at least 16-years-old, but you have to be at least 18 to get the Moderna vaccine and the Johnson & Johnson vaccine.

Those age limits, set by the Food and Drug Administration, are based on how the three companies conducted their pharmaceutical trials.

Before getting emergency-use authorization, both Moderna and Johnson & Johnson had limited their trials to people ages 18 and older, while Pfizer was testing with children as young as 12.

According to the New York Times, both Pfizer and Moderna are now testing with children ages 12 and older but it may take until summer before the vaccines receive approval for this younger age group.

Johnson & Johnson also plans to test its vaccine with younger patients but is further behind.

You might be wondering, if Pfizer was testing with children as young as 12, why did the FDA set the age minimum to 16?

The Pfizer vaccine got approval on Dec. 11 after testing its vaccine with 44,000 participants, but the company didn’t actually have enough children between the ages of 12 and 15 participating until January, according to CNBC.

CNBC reports that the study enrolled 2,259 kids between the ages of 12 and 15, and its entry on a government website for clinical trials shows the company is no longer recruiting patients to take part in the trial.

On Dec. 18, Moderna became the second COVID-19 vaccine to receive emergency use authorization after testing its vaccine with 15,419 participants aged 18 years and older across three clinical trials.

According to the emergency-use authorization, “Safety and effectiveness have not been assessed in persons less than 18 years of age. Emergency Use Authorization of Moderna COVID-19 Vaccine does not include use in individuals younger than 18 years of age.”

On Feb. 27, the FDA approved the Johnson & Johnson vaccine after testing with 43,783 participants. Similar to Moderna, Johnson & Johnson also only tested its vaccine with people ages 18 and older.