Why you should keep wearing a mask even if you are vaccinated

The United States Centers for Disease Control and Prevention (CDC) recommended that all people in areas with high rates of COVID-19 infection wear masks in closed public spaces, regardless of their vaccination status .

This is a turnaround from the CDC advice of May 2021, according to which fully vaccinated people could leave their masks at home , and brings the US guidelines more in line with the recommendations of the World Health Organization ( WHO) .

Peter Chin-Hong, an infectious disease physician at the University of California, San Francisco, explains the scientific data that underpins these changing messages.

What scientific data supports the use of masks after vaccination?

Masks help stop the spread of the coronavirus . They are a literal layer between people and any viruses in the air, and they can help prevent infection.

The reason public health officials are calling for more use of the mask is that there is clear and growing evidence that - while rare - COVID-19 infections can occur in people who are fully vaccinated . This is especially true for emerging variants that are of concern . The good news is that COVID-19 infection, if it occurs, is much less likely to cause serious illness or death in vaccinated people.

Some conditions make infection more likely in a vaccinated person: more viruses circulating in the community, lower vaccination rates, and more highly transmissible variants.

If vaccinated people can become infected with the coronavirus, they can also spread it . Hence the CDC's recommendation that vaccinated people wear masks in closed public spaces to help stop the transmission of the virus.

Where will the guidelines apply?

The CDC recommendation on masks targets areas of the United States with more than 50 new infections per 100,000 residents or where more than 8% of tests were positive in the previous week. By the CDC's own definitions , "substantial" community transmission is 50 to 99 cases of infection per 100,000 people per week, and "high" is 100 or more.

Los Angeles County, for example, far surpassed that mark in mid-July, with more than 10,000 coronavirus cases per week.

Using these criteria, the CDC guidelines were applied to 63% of US counties on the day they were announced.

^{Wearing masks is most protective of the unvaccinated - including children. Paul Bersebach / MediaNews Group / Orange County Register via Getty Images}

Who do the recommendations to wear masks really protect?

The recommendation that fully vaccinated people continue to wear a mask is primarily intended to protect the unvaccinated - including children under 12 who cannot yet be vaccinated in the US. The CDC further recommends the use of masks for vaccinated individuals with unvaccinated family members, regardless of local community transmission rates.

Unvaccinated people are at substantially higher risk of becoming infected with and transmitting SARS-CoV-2 , and of developing complications from COVID-19.

How do new variants like the delta change the scene?

Preliminary data suggests that increasing variants such as delta may increase the chance of infections in people who only received the first dose of the vaccine. For example, one study found that a single dose of Pfizer's vaccine was only 34% effective against the delta variant, compared to 51% against the original alpha variant, in terms of preventing symptomatic disease.

The data is more reassuring for those who have been fully vaccinated. After two doses, the Pfizer vaccine continues to provide strong protection against the delta variant, based on actual data from Scotland and other countries; and in preliminary studies from Canada and England , researchers observed only a "modest" decrease in efficacy against symptomatic disease, from 93% for the alpha variant to 88% for the delta .

However, other recent preliminary reports from highly vaccinated countries like Israel and Singapore are sobering. Before the delta variant became widespread, from January to April 2021 , Israel reported that Pfizer's vaccine was 97% effective in preventing symptomatic disease. Since June 20, 2021, with the delta variant circulating more widely, Pfizer's vaccine has been only 41% effective in preventing symptomatic disease, according to preliminary data reported by Israel's Ministry of Health in late June. July.

An analysis using government data from Singapore showed that 75% of recent COVID-19 infections occurred in people who were at least partially vaccinated - although most of them were not seriously ill.

^{Some US experts were concerned that the official message that vaccinated should wear masks might deter the unvaccinated from seeking vaccines. Jeff Gritchen / MediaNews Group / Orange County Register via Getty Images}

Yet across all reports and studies, vaccines are still very good at preventing hospitalizations and serious illness due to the delta variant - possibly the outcomes that matter most to us.

All of these emerging data support the WHO global recommendation that even fully vaccinated people continue to wear masks. Most of the world continues to have low vaccination rates and uses a series of vaccines of varying efficacy , and countries have different loads of circulating SARS-CoV-2 viruses.

Given that the case counts and contagion figures in the US go in a direction that public health officials consider wrong, it makes sense for the CDC to modify its recommendations for wearing masks to be more conservative.

What conditions in the United States justify the use of masks (again)?

It makes sense that the CDC did not immediately change its recommendations to conform to the June WHO guidelines. With a high overall vaccination rate nationwide and a low COVID-19 hospitalization and death burden, the United States has a very different COVID-19 outlook than most of the world .

Additionally, some experts were concerned that the official message that vaccinated should wear masks might deter the unvaccinated from seeking vaccines.

But, as President Joe Biden said on July 27, " new research and concern about the delta variant " is behind the CDC's change in mask use recommendations.

In some places, there is a further increase in community transmission, including among vaccinated people . Preliminary new research, not yet reviewed by experts, suggests that the delta variant is associated with a viral load 1,000 times higher in patients than that seen with older strains. And the first reports show that people infected and vaccinated with the delta variant can carry as high an amount of virus as non-vaccinated people , which in turn can infect other people.

Changes in the recommendations do not necessarily mean that the old ones were wrong, but rather that the conditions have changed. The conclusion? Masks help reduce the transmission of the coronavirus, but vaccines are still the best protection.

^{This article is republished from The Conversation under a Creative Commons license. Read the original article .}