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Coronavirus delta variant: What you should know

The delta variant is highly contagious, responsible for a growing number of hospitalizations and the dominant variant in the US. Here's what you should know.

- 07:20

The CDC now considers delta a "variant of concern."

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For the most up-to-date news and information about the coronavirus pandemic, visit the WHO and CDC websites.

The novel coronavirus is, unfortunately for us, good at doing what viruses do best: mutating in order to become more efficient and infect more people. The delta variant, a strain first documented in India, has caused devastation in the country and is now the dominant strain in the US and many other parts of the world.

The delta variant's growth across the globe has 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 states of New South Wales, Victoria and South Australia back into strict lockdowns. In the US, an increase in COVID cases has caused places such as Los Angeles County to reinstate mask mandates.

The strain is about 60% more transmissible than the alpha variant 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.

The US is fortunate to have great vaccine availability, and 68.4% of the adult population has received at least one dose of a COVID-19 vaccine, according to July 21 data from the Centers for Disease Control and Prevention. However, as the delta variant spreads and many pockets of the country have low vaccination rates, experts worry about a surge of hospitalizations and deaths. 

"Over 97% of people who are entering the hospital right now (with COVID) are unvaccinated," CDC director Dr. Rochelle Walensky said Friday.

All three vaccines available in the US are said to have excellent efficacy against severe disease caused by COVID-19. But how do they fare against the more contagious variant? Research results have been 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 the delta variant 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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Pfizer booster? How Moderna and Pfizer compare 

Pfizer announced that the company is working on a booster specifically for the delta variant, citing real-world data from Israel that shows that Pfizer's two-dose regimen is less effective against infection from delta compared to previous variants -- down to 64%, though the same data found that the vaccine is still 93% effective at preventing severe disease. In a strange twist, the CDC and Food and Drug Administration issued a joint statement in early July that said (without calling out Pfizer specifically) fully vaccinated Americans do not need a booster at this time.  

Earlier research from late May by the UK body Public Health England found that Pfizer is more effective than what Israel's data shows, and that two doses of Pfizer's or AstraZeneca's COVID-19 vaccine are very 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, against 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's 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. 

Meanwhile, in the US, Moderna announced at the end of June 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 particularly promising.

J&J and the importance (or lack thereof) antibodies 

Since delta first made its appearance, many who got the Johnson & Johnson vaccine, the US's only single-dose vaccine, started wondering if their protection against COVID-19 would last, and whether they would need a booster. 

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.)

But booster shots are not recommended for anyone right now, including those who got J&J, and the company announced recently that its shot is effective against delta, possibly even more effective compared to the beta variant in South Africa where previous research was done. Earlier this month, research from the South African Medical Research Council 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%). 

In a study that came out July 19 and hasn't yet been peer-reviewed, researchers found that Johnson & Johnson's vaccine elicited a weaker antibody response to the delta variant when compared with Moderna and Pfizer's vaccines  (which also showed lowered antibody responses to delta). This fueled news that Johnson & Johnson's vaccine isn't effective because of a reduction in antibodies.

But the problem with measuring antibodies is that it doesn't show the whole picture of immunity or how well a vaccine is working, experts say. In fact, the FDA issued an alert in May that "antibody tests should not be used to evaluate a person's level of immunity or protection from COVID-19 at any time, and especially after the person received a COVID-19 vaccination." 

While easy to measure after infection, other factors of immunity, such as T-cell responses, offer a better look into our body's memory in how to fight infection. A study from early June found that while Johnson & Johnson's antibody response was reduced against coronavirus variants (not including delta), the vaccine's T-cell response was "largely preserved." 

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 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.