The airline announced a change to its pet policy on Thursday that bans animals under four months from the cabin. The move is “to further ensure the well-being of our employees and customers while accommodating passengers with disabilities,” United said in a statement.

Young pets often have not received vaccinations, making them a risk to other passengers and employees.

The new policy is set to go into effect today.

Additionally, United is limiting emotional support animals to flights under eight hours. The airline says that it has “seen increases in onboard incidents on longer flights involving these animals, many of which are unaccustomed to spending an extended amount of time in the cabin of an aircraft.”

Emotional support animals are now limited to just dogs and cats on United flights and permitted service animals are limited to dogs, cats, and miniature horses.

United’s decision follows a similar policy change made by Delta in December. Delta also banned animals under four months from the cabin due to USDA vaccination requirements and no longer allows emotional support animals on flights longer than eight hours.

In August, Southwest limited emotional support animals to just dogs, cats, and occasionally miniature horses after a biting incident involving a dog on a flight.