People with certain health conditions that make them moderately or severely immunocompromised can receive a fourth dose of the Covid-19 vaccine, according to updated guidelines from the US Centers for Disease Control and Prevention (CDC).

The CDC authorized a third dose for some immunocompromised people aged 18 and over last August. He said a third dose, rather than a booster dose — the CDC distinguishes between the two — was necessary because an immunocompromised person may not have a full immune response with the first two doses.

A study by Johns Hopkins University this summer showed that immunocompromised people who were vaccinated were 485 times more likely to end up in hospital or die from Covid-19 than most vaccinated people.

According to the Centers for Disease Control and Prevention, immunocompromised people who were fully vaccinated were responsible for about 44% of emergencies requiring hospitalization. People who are immunocompromised are also more likely to pass the virus on to people they have been in close contact with.

The CDC estimates that about 9 million people living in the United States, or about 2% of the population, fall into this category.