“The war has changed,” read an internal CDC document, warning that Delta variant infections are more severe, and the variant is far more contagious than the other ones we’ve seen so far.

Vaccination is still our best weapon against the virus — it makes us less likely to get the disease in the first place, and far less likely to suffer any serious symptoms if we do get it. But while vaccinated people are safe themselves, they can still pass on the virus if they do get it. A CDC study found that the Delta variant produced similar amounts of virus in vaccinated and unvaccinated people once infected.

“High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus,” Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said in a statement Friday.

As cases surge across many parts of the US (and the world), this revelation triggered a change in guidelines; now, the US federal guidance is that most people should wear masks indoors even if they are vaccinated. SciLine asked three experts what they think about this guidance, and about vaccinated people wearing masks indoors in general.

“Since we don’t know ahead of time which areas have high COVID-19 transmission, it is best to err on the side of caution and wear masks whenever you are indoors if you are concerned about being infected with—or transmitting—COVID-19 to others. Those who spend time around those who are unvaccinated, or those who are vaccinated but potentially at high risk, like people who are immunocompromised, should be especially cautious,” says Dave O’Connor, a Professor of Pathology and Laboratory Medicine at the University of Wisconsin-Madison.

We’re still playing a game of odds against COVID-19. The best you can do, by far, is getting vaccinated — the vast majority of infections happen in unvaccinated people, but those who are vaccinated can still help protect those around them.

According to an anonymous federal health official quoted by the Washington Post, vaccinated people can transmit the virus, in rare instances. If this is the case, it’s better to play it safe and have people wear masks in crowded indoor places, experts seem to agree.

“Fully vaccinated people should wear masks in any indoor setting where there are large groups of people whose vaccination status is unknown. Put on a mask when entering a grocery store, a crowded bar or restaurant, when using public transportation, or when attending an indoor event where staying greater than six feet apart is not possible. Those are potential areas of high transmission. Also, before travel, check the CDC website to see if where you’re going has a high COVID-19 infection rate. In those areas, always wear a mask unless you’re outdoors or indoors with family and friends whom you know are all vaccinated. Regardless of region, always wear a mask when you enter any health care or senior care facility,” says Deborah Fuller, Professor of Microbiology, University of Washington School of Medicine.

This is likely not the last time the guidance will change. As the situation continues to change and we learn more and more about the virus and new data comes in, scientific and medical guidance will likely be tweaked again.

“When the CDC put in their mask guidance on May 13 that the vaccinated need not mask indoors, they were being consistent with the data that vaccines were very effective and reduced transmission.  However, the Delta variant is much more transmissible, leading to more mild breakthroughs among the vaccinated in areas of high community transmission and high cases and hospitalizations among unvaccinated adults. The reason for the increased mild breakthroughs is likely due to higher circulating virus making even vaccinated people likely to be exposed.  Therefore, the CDC has changed to a more metric-based set of mask guidance, rather than a single set of guidelines for the country, in response to a new variant with increased transmissibility. If one lives in an area with high rates of circulating virus (substantial or high transmission), the CDC is recommending that even vaccinated people should mask indoors, which I think is prudent until the Delta wave subsides,” says Monica Gandhi, Infectious diseases doctor and Professor of Medicine at the University of California, San Francisco.

Current estimates from the CDC internal document show that vaccines reduce the risk of severe disease by at least a factor of 10, and infection by a factor of 3. Previous data showed that vaccines are 95% effective in preventing symptomatic COVID-19 in Israel, 90% effective in first-line responders and health care workers in the US, and 97.4% effective against severe disease, even against the B.1.351 and B.117 variants, in Qatar. 

The US, like other developed countries, is experiencing a surge of new cases despite relatively high vaccination rates — the number of cases hasn’t been this high since February 2021 (although the number of hospitalizations remains relatively low compared to previous waves). Experts are also concerned with plateauing vaccination rates.

The coronavirus pandemic is still far from over, and cases continued to rise in over 100 countries in the past week. While even developed countries are struggling with the new wave, for many developing nations, the situation is even more dire. Globally, less than 30% of the population has received at least one shot of the COVID-19 vaccine, and with insufficient access to vaccines, many developing nations will continue to struggle. To add even more pressure, developed countries that started vaccination early are already pondering the acquisition of booster shots, which will add even more pressure to global supply. Ultimately, as long as the virus is allowed to run uncontrolled across the world, we’ll continue to face the risk of new, dangerous variants emerging. The world is slowly headed in the right direction pandemic-wise, but we’re not at the finish line just yet.