The CDC now considers delta a “variant of concern.”

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The coronavirus is, unfortunately for us, good at doing what viruses do best: mutating in order to become more efficient and infect more people. The newest mutation to cause concern is the delta variant, a strain first documented in India, which has since caused devastation in the country. 

The delta variant’s growth across the globe has recently caused some countries in the Asia-Pacific region to reimpose lockdowns and other restrictions governments had lifted, The New York Times reported in June. Notably, COVID-19 outbreaks in Australia, fueled by the delta variant, have forced the cities of Sydney, Perth, Brisbane and Darwin back into strict lockdowns.

At the end of May, the World Health Organization renamed the coronavirus variants of concern with letters of the Greek alphabet. What is now called the delta variant has moved from merely being identified as present in the US to accounting for more than 20% of US COVID-19 cases, Dr. Anthony Fauci, the president’s chief medical advisor, said on June 22 at a White House press briefing. The number of COVID-19 cases caused by the delta variant are doubling every two weeks in the US, Fauci said. 

Helix, a California-based genomics company, told Reuters that the delta variant is responsible for around 40% of COVID-19 cases in the US, based on the company’s model.  

The US is fortunate to have great vaccine availability, and 66.7% of the adult population has received at least one dose of a COVID-19 vaccine, according to July 1 data from the Centers for Disease Control and Prevention. But even with an increasing number of people vaccinated against COVID-19, the delta variant’s appearance in the US is concerning. 

The strain is about 60% more transmissible than the alpha variant strain first documented in the UK, British epidemiologist Neil Ferguson told reporters in June, and it’s suspected to carry the potential for more severe disease. In a Scottish study published on June 14, researchers found that the delta variant carried double the risk of hospitalization compared with the alpha variant.

Experts in the US worry because the delta variant is following the same course it took in the UK, where it started off small, but eventually became the dominant strain in the country. In a White House briefing last week, Dr. Rochelle Walensky, director of the CDC, said she expects the delta variant to become the dominant strain in the coming weeks.

The three vaccines available in the US have all proved to have excellent efficacy against severe disease caused by COVID-19. But how do they fare against the more contagious variant? Research is promising, and scientists still urge vaccination as the way out of this deadly pandemic.

Delta’s side effects 

In mild cases, the delta variant may show up a little differently. Tim Spector, British epidemiologist and co-founder of the ZOE COVID Symptom study in the UK, says that since May, the top symptoms of COVID-19 being recorded in the app have changed. He says the No. 1 symptom of COVID-19 is now headache, followed by sore throat, runny nose and fever — not the more “classic” COVID-19 symptoms. Coughing is now the fifth-reported symptom.

“We don’t even see loss of smell coming into the top 10 anymore,” he says. “This variant seems to be working slightly differently.” 

Spector says these changes — which appear to be linked to the delta variant — may cause people to continue going out, thinking they have a common cold instead of COVID-19.

News of COVID-19 causing a black fungus in the sinuses and brain and hearing loss, among other serious symptoms, in COVID-19 patients from India may be alarming and lead you to believe delta causes new and scarier side effects than older strains of the novel coronavirus. The reality is COVID-19 from all strains and forms is a formidable disease, with a myriad of side effects that can last months after infection in some people. As Business Insider reported, there isn’t enough data at this time to support the conclusion that the delta variant causes these atypical symptoms.


The WHO labels “variants of concern” and “variants of interest” using letters of the Greek alphabet. The term means they have been associated with an increase of transmissibility, increase in virulence or change in clinical presentation, a decrease in effectiveness of public health measures, or some combination of the above, according to the WHO. 

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How Pfizer, Moderna and J&J compare 

Research from late May by the UK body Public Health England found that two doses of Pfizer’s or AstraZeneca’s COVID-19 vaccine are effective against the delta variant — about 88% (Pfizer) and 60% (AstraZeneca). Although this study shows that two doses are about 5 and 6 percentage points less effective, respectively, with the delta variant than the alpha strain, the study also found that both vaccines had about 33.5% effectiveness against delta after one dose of either AstraZeneca or Pfizer — considerably less protection than two doses of either vaccine. 

The Scottish study published in June also confirms Pfizer and AstraZeneca’s effectiveness against infection from the delta variant. Researchers from this study say AstraZeneca gave 60% protection two weeks after the second dose, while Pfizer gave 79% protection two weeks after the second dose. 

On July 5, Israel’s Ministry of Health said that Pfizer’s protection against COVID-19 infection from delta has fallen to 64%, Haaretz reported, down from 91.2% efficacy in March. The ministry said that Pfizer is 93% effective at preventing hospitalizations and severe symptoms, which is still highly effective. 

Meanwhile, in the US, Moderna announced Tuesday that its vaccine appears to be effective against all current coronavirus variants, including delta. Because Moderna and Pfizer are so similar, it seems likely it would perform similarly, but this is extra promising.

As for Johnson & Johnson (the US’s single-dose vaccine), research from the South African Medical Research Council published Thursday found that breakthrough COVID-19 cases in health care workers who got a Johnson & Johnson shot were mild in 94% of cases (moderate cases accounted for 4%, severe cases 2%). The studies were done when beta and delta variants were circulating. 

These findings were published days after some experts, including Stanford professor Dr. Michael Lin, say that those who got a J&J single-dose vaccine may need a booster sooner than their Pfizer- or Moderna-receiving counterparts in the face of the delta variant. On Twitter, Lin said that while the CDC may be waiting before making an official decision about booster shots, receiving an mRNA vaccine after getting the Johnson & Johnson will likely be safe, and pointed out a study done in Spain that suggests benefits to mixing Pfizer and AstraZeneca vaccines. (AstraZeneca is a viral vector vaccine, like Johnson & Johnson.)

Moving forward

The expert advice does not seem to change as the coronavirus does — getting vaccinated and gaining immunity against COVID-19 is the surest way to prevent severe disease caused by any variant. When fewer people get infected with COVID-19, the coronavirus has less room and fewer hosts within which it can mutate and produce new variants.

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.