Philadelphia health officials said Thursday they now “strongly recommend” that everyone — including the fully vaccinated — wear masks inside public places.

Here’s what you need to know:

Why is the city recommending indoor masking again?

Philadelphia health officials are recommending indoor masking because the highly contagious delta variant has led to a spike in COVID-19 cases among unvaccinated individuals. The vaccines are effective against the delta variant, but public health officials are concerned about an increase in hospitalizations among children who are too young to be vaccinated.

Where are masks required?

Philadelphia is only recommending people wear masks indoors. However, masks are still required in health-care settings, public transit, schools, child-care, indoor camps, and congregate living facilities.

Is the new recommendation just in Philadelphia?

Chester, Montgomery, Delaware, and Bucks Counties have no plans to change mask guidance at this time. New Jersey Gov. Phil Murphy said the state also has no plans to reimpose mask guidelines or other restrictions.

Several counties in California, including Los Angeles County, have updated their guidelines to recommend or require masks regardless of vaccination status.

The World Health Organization still encourages masking and the Washington Post reports that federal officials are debating an update to the federal Centers for Disease Control and Prevention’s guidance.

The CDC said vaccinated people don’t need masks anywhere. Why the change in Philly?

Since there is no way to know who is vaccinated, the health department thinks it’s better to “normalize” mask-wearing for everybody. It would be too burdensome to ask businesses to check vaccine status when customers enter.

How bad is the delta variant compared with other strains of the virus?

The delta variant, which originated in India, is about 60% more transmissible than the alpha variant, also known as the U.K. variant, according to a study by researchers in London. There has been an increase in children contracting the delta variant and some evidence that it can lead to more severe illness than the original strain. So far, the three vaccines authorized for use in the United States are providing good protection from the original strain and variants such as delta.

Genetic sequencing suggests the delta variant is in the Philadelphia area.

How many vaccinated people are getting COVID-19?

Instances of vaccinated individuals contracting COVID-19, called “breakthrough cases,” are uncommon. As of July 12, the CDC has tracked 5,492 breakthrough cases among 160 million vaccinated individuals — roughly 0.003% of vaccinated people have been hospitalized or died of COVID-19.

Among those who have contracted the virus after getting vaccinated, the illness was, in most cases, more mild. Hospitalization and death from COVID-19 after vaccination have been very low.

If vaccinated people are getting COVID-19, why should I bother getting the vaccine?

There is a much higher risk of contracting COVID-19 without a vaccine than with one. The more people who are vaccinated, the harder it will be for the virus to continue to spread and mutate into new, potentially more contagious variants.