The Centers for Disease Control and Prevention is recommending fully vaccinated people wear masks indoors again in some areas — including schools — it announced during a telebriefing Tuesday. This is different from its guidance earlier in July that said it’s safe for teachers and students who are fully vaccinated to skip the face masks. So why the sudden change? A spike in COVID-19 cases across the US caused by the highly contagious delta variant has resulted in the CDC updating its guidance for mask wearing.

The World Health Organization continues to recommend that to slow the rapidly spreading delta variant, everyone should continue to wear face masks in crowded areas, even people who are past the two-week mark after receiving their second vaccine dose, for example from Pfizer, Moderna or AstraZeneca.

Both the WHO and CDC agree that people who aren’t vaccinated should continue to wear face masks. But given the high rate of protection that vaccinated people have against the delta variant and other COVID strains, why are some medical experts in conflict about a barrier over your nose and mouth? We continue to update this story.

CDC guidance: Fully vaccinated should continue wearing masks in some areas

The CDC on Tuesday changed its guidance for mask wearing to say that vaccinated people should continue wearing masks indoors in certain areas of the country. This includes areas where COVID-19 cases are on the rise. It also changed the guidance for K-12 schools, saying everyone should wear a mask, regardless of vaccination status. Earlier this month, it said fully vaccinated teachers and students don’t need to wear masks inside the school.

Originally, the guidelines said those who are fully vaccinated can «resume activities without wearing masks or physically distancing» except where required, such as in planes and businesses. CDC Director Dr. Rochelle Walensky previously stuck to that guidance on NBC’s Today show on June 30, saying fully vaccinated people are protected from the delta variant.

The CDC warned CNN that there’s still a small chance fully vaccinated people can become infected with the new variant if they’re exposed. We’ve asked the CDC for comment.

What is the CDC’s new guidance for kids and teachers?

Those who aren’t vaccinated — ages 2 and up — need to wear a mask while inside but don’t have to when they’re outside. For fully vaccinated teachers and students, the CDC recommends continuing to wear masks inside the school building.

For child care programs, regarding schools with children younger than 12, universal mask use can be implemented, as children this age aren’t eligible for the COVID-19 vaccines yet. Mask mandates can also go into effect if coronavirus transmission is high in that community.

For schools that can’t provide adequate physical distancing, the CDC says «layered COVID-19 prevention strategies» should be put in place to protect kids and teachers who aren’t fully vaccinated.

Read more: In-person learning should be a priority for schools this fall, CDC says

Why are experts at odds over masks for vaccinated people?

In June, WHO officials repeated a longstanding recommendation that everyone should wear masks to stem the spread of the virus. Meanwhile, the CDC until Tuesday had been continuing to uphold its guidance that not all fully vaccinated Americans need to wear masks indoors or maintain social distancing. White House Chief Medical Adviser Dr. Anthony Fauci said that those who’ve had two doses of the vaccine should still «go the extra step» of wearing a mask when traveling to places with low vaccination rates.

A few days after the WHO recommended that people who have received the COVID-19 vaccine should continue wearing a face mask, Fauci explained why the advice differed from the CDC’s. «There’s a reason for that,» he said in a virtual White House press briefing. «The WHO is responsible for the planet as a whole. It’s different in the world in general from here in the United States.»

So, while the WHO is monitoring the pandemic around the world, with a majority of people unvaccinated, the CDC is responding to the situation in the US, where the vaccines have been shown to work quite effectively against hospitalization from the delta variant. Only 13.9% of the global population is fully vaccinated, according to vaccine tracker site Our World in Data. The US nearly quadruples that number: 49.7% of Americans are considered fully vaccinated as of July 4, according to the Mayo Clinic.

For Fauci and the CDC, the focus is on getting a greater number of Americans vaccinated in the first place. On July 4, Fauci said that 99.2% of COVID-related deaths last month involved unvaccinated people.

What is WHO’s position on mask wearing?

Wearing a mask consistently continues to be important, even for people who are vaccinated, a WHO official said during a press briefing in late June. «People cannot feel safe just because they had the two doses. They still need to protect themselves.»

Wearing masks in public places is essential to keep people from inhaling particles that will cause them to become sick, Dr. Maria Van Kerkhove, an American infectious disease epidemiologist, said during the same briefing.

While the Pfizer and Moderna vaccines are showing robust protection against variants, «breakthrough» infections may still sporadically occur. In a case in early June, a fully vaccinated woman in Napa, California, died from COVID-19. She was over 65 and was reported to have underlying medical conditions.

Why is the coronavirus delta variant serious?

The delta variant is the latest of new coronavirus variants and is the most contagious of the variants identified so far, according to the WHO. The delta variant has been identified in 96 countries so far and is spreading among the unvaccinated population, the organization said during the press conference.

«The delta variant is currently the greatest threat in the US to our attempt to eliminate COVID-19,» Fauci said during a White House press briefing on June 22.

Will there be new mask mandates or tighter restrictions?

Some countries have already begun tightening COVID-19 restrictions. For instance, four cities across Australia have returned to lockdown, including Sydney’s state of New South Wales. In the US, Los Angeles County is strongly recommending face masks indoors, regardless of whether someone has been vaccinated, though masks aren’t required in most places. Some countries within Africa and Asia — for instance, South Africa and Malaysia — have also moved into lockdown due to the delta variant spreading.

Some places never loosened mask mandates, such as airports and airlines. In the US, other public transportation facilities require masks, such as buses and trains.

For more information, here’s what you need to know about «long COVID» and how it’s treated. Also, read up on these COVID-19 vaccine side effects and important dos and don’ts of getting your COVID-19 vaccine.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.