Is a face shield as effective as a face mask?

Question

On a CDC web page dated August 7, 2020, they warned against the use of face shields as an effective method to stop COVID-19, due to a lack of evidence.

There is currently not enough evidence to support the effectiveness of face shields for source control. Therefore, CDC does not currently recommend use of face shields as a substitute for masks.

They go on to give examples where face masks aren't suitable, and advice on how to use as face shield in those circumstances.

An ABC7 News article from July 4, 2020 gives similar advice from the CDC:

Officials at the Centers for Disease Control and Prevention were asked if shields are as good as masks.

"I don't think we have data yet that a shield is better or worse," Dr. Anne Schucat with the CDC said. "We are recommending for the consumer, we are saying the cloth face mask is what we are recommending."

They also quoted an expert saying that face shields were as effective:

Some users claim face shields are just as good at keeping you safe as the cloth alternative.

"I think face shields are a great option. One great advantage this time of year is that they aren't as hot as a face mask is," Dr. Michael Edmond, Professor of Infectious Diseases at University of Iowa, said.

A university health clinic looked at the question with an article from July 23, 2020.

They explained that the question of comparing face shields to face masks was complicated due to the differing effectiveness of different masks.

There has not been a study comparing face shields to masks, and doing such study would be challenging since mask materials vary greatly in the community.

However, Veltman says research has found that face shields are beneficial if someone cannot wear a mask.

I feel that the face-shield is just as effective.

Are face shields as effective as face masks to prevent SARS-CoV-2 being spread?

Answer 1

In addition to all the (non-Covid) research which I posted to Med.SE a while ago and I'll not wholly repeat here, but here's a snippet of a 2014 paper, which concludes (using a cough simulator):

In the period from 1 to 30 minutes after a cough, during which the aerosol had dispersed throughout the room and larger particles had settled, the face shield reduced aerosol inhalation by only 23%. [...] Face shields can substantially reduce the short-term exposure of health care workers to large infectious aerosol particles, but smaller particles can remain airborne longer and flow around the face shield more easily to be inhaled. Thus, face shields provide a useful adjunct to respiratory protection for workers caring for patients with respiratory infections. However, they cannot be used as a substitute for respiratory protection when it is needed.

... we have a more recent Swiss "natural experiment" in which workers wearing just shields got infected with Covid-19, but those wearing masks didn't.

The infections were reported at a hotel in the Graübunden region, where several employees and at least one guest tested positive for the virus. Local health officials determined that all those infected had been wearing only plastic face shields as protection. No one who had worn a face mask was infected, according to officials.

More coverage of the same Swiss event:

Rudolf Leuthold, head of the cantonal health department in Graubünden, said the face shields were the common denominator in infections.

“It has been shown that only those employees who had plastic visors were infected. There was not a single infection among employees with a mask.”

---

More recent research mentioned in a BBC article:

Where things get difficult is with what happens to the aerosols when wearing a plastic shield.

“Nearly all of the aerosols were coming around the side of the face shield and reached nearly the same distances as without wearing anything,” says Echternach. These results are still to be published, but Echternach says they should serve as a warning for anyone relying on face shields alone to keep them safe as pandemic lockdowns are eased.

“They are certainly not effective when you are in close contact with someone,” he says.

The same BBC source links to an Israeli study (still in preprint) finding/recommending otherwise. I'm rather skeptical of it, given all the other evidence to the contrary... This paper found that a face shield reduces inhaled particles by orders of magnitude more than an N95 mask [fig 4] which is an astounding claim! It's not totally clear to me what they mean by "total maximal particles number" though. I could also not figure out from the Israeli paper how long each post-simulated-cough run lasted--something that's clearly stated in the 2014 paper, for instance.)

Answer 2

From an article in the New York Times:

Face Shields and Valved Masks Offer More Comfort but Less Protection

While any face covering is better than nothing, clear plastic shields and masks with exhale valves allow large plumes of particles to escape and may also expose the wearer to more germs.

Face shields and valved masks — two options many people find more comfortable than cloth face coverings — appear to be less effective at blocking viral particles than regular masks, a new study shows.

The Centers for Disease Control and Prevention had already stated that clear plastic face shields and masks equipped with vents or valves are not recommended, because of concerns that they don’t adequately block viral particles. But the new research, which uses lasers to illuminate the path of coughs, offers a striking visual demonstration of how large plumes of particles can escape from behind a face shield or vented mask.

“I think these visualizations are really powerful for helping the general public to see and understand what’s happening,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who studies airborne particles but who was not involved in the research.

...

But the new research, published in the journal Physics of Fluids, shows that face shields alone and vented masks allow large plumes of particles to escape, putting those around you at risk. And while the research did not specifically look at the level of protection the shields and masks offer the wearer, it does suggest that people who use them may also be more vulnerable to exposure than if they wore a regular mask. Valved masks are a particular concern — some of the nonmedical vented masks the researchers used had faulty valves, suggesting that some people may be walking around with open valves — essentially large holes — in their masks.