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The Delta Coronavirus Variant Warrants Mask Wearing in Crowded Indoor Spaces. Recent transmission events in Sydney, Australia illustrate why.
In two of the Delta variant cases in Sydney, the moment of transmission was captured on CCTV. In the first, a man in his 50s caught the virus by passing through the airspace of an infected limousine driver at Myer at Bondi Junction. They never touched, but were several inches away for a few seconds.
In the second instance, a woman in her 70s was sitting outside at Belle Cafe in Vaucluse when the same driver — the cluster’s primary case — was inside the cafe. Genomic sequencing of her infection confirmed an exact match to the driver's.
The Delta variant causes a higher viral load that is so much more easily spread that even this type of fleeting contact, particularly in indoors, poorly ventilated spaces, can cause infection.
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The Delta Variant is Growing Quickly in the U.S.
Just two weeks ago, the Delta variant was only ~10% of all sequenced cases; it's now estimated to be over 20% of sequenced cases. In the UK, the Delta variant is already 98% of sequenced cases, having almost completely taken over the surge there. The UK's numbers are some of the most reliable, since the UK sequences about 30% of all cases (the U.S. only sequences 5% of cases).
It's instructive to look at this chart, kindly shared by epidemiologist Dr. Eric Feigl-Ding, depicting the relative growth of the Delta variant in the days after the 5th case was reported–it grew *much* faster in England than other variants, and that's what we can expect in the more poorly vaccinated areas of the U.S., particularly outside the Northeastern U.S. and outside the heavy hit/highly vaccinated areas of California such as LA and the Bay Area.
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Vaccination is Key, But It Depends on the Vaccine, and It's Not Perfect
A single dose of either the Pfizer or AstraZeneca vaccine reduces a person's risk of developing COVID-19 symptoms by only 33% (it reduced the risk by 50% for the Alpha variant). A second AstraZeneca dose increases protection against the Delta variant to 60% (vs. 66% against Alpha), while Pfizer’s second dose is 88% effective at reducing the risk of developing COVID symptoms (compared to 93% against Alpha).
Notably, Israel, the most vaccinated country in the world, has seen some coronavirus outbreaks at schools connected to recent foreign travel, and in addition to the 44 kids who tested positive for the Delta variant, several fully vaccinated adults at the schools were also infected. Israel has used the Pfizer vaccine for its vaccinations, not the less effective AstraZeneca vaccine.
Read: Which Travel Destinations are the Most Vaccinated?
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Given the rapid increase of the Delta variant in the U.S., we're glad that a face mask mandate remains in effect on flights, in airports and train stations, and on trains until at least September 13, 2021. And while we enjoy going mask-free outdoors, we plan to continue to wear face masks in all crowded indoor spaces, especially poorly ventilated ones, based on the rapid spread of the Delta variant.
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