Delta variant viral load 1,000 times higher than original coronavirus strain

BEIJING - The fast-spreading delta variant of the novel coronavirus has been gaining traction worldwide since it was first detected in India, and now researchers may have an idea of why: Its viral load is 1,000 times higher than the original strain of the virus.

The delta variant is a mutated version of the novel coronavirus that spreads more easily than other strains. It now accounts for an estimated 83% of COVID-19 cases in the United States as it continues to surge largely among unvaccinated populations, officials said. 

Viruses constantly mutate, and most changes aren't concerning. But there is a worry that some variants might evolve enough to be more contagious, cause more severe illness or evade the protection that vaccines provide.

A recent study published on July 7 and led by Chinese epidemiologist Jing Lu at the Guangdong Provincial Center for Disease Control and Prevention in Guangzhou, China shed light on why the delta variant is of grave concern to the world’s health care system and why it is more transmissible than other mutations. 

Why is the delta variant more contagious?

Study authors noted that one characteristic that makes the variant so worrisome is its high viral load.

Researchers found that the delta variant contains 1,000 times more viral material than that of the original novel coronavirus variant that infected much of the global population during the onset of the global pandemic last year. 

RELATED: Delta variant: What to know about the spreading coronavirus strain

This means the delta variant can replicate at a much faster rate than the original strain, making the mutation much more infectious, according to the study. 

When a person becomes infected with the delta variant, the mutation is shedding significantly more viral material, making it harder to suppress and easier to infect others.

Researchers also found that on average it took approximately four days for the delta variant to reach detectable levels using a standard COVID-19 test kit compared with the six days it took for the original coronavirus strain to be detected. 

Experts believe the delta variant spreads more easily because of mutations that make it better at latching onto cells in human bodies. On its website, the U.S. Centers for Disease Control and Prevention notes "increased transmissibility" with the delta variant and the potential for it to make certain monoclonal antibody treatments less effective. 

Is the delta variant more deadly?

It's not clear yet whether the variant makes people sicker since more data needs to be collected, Dr. Jacob John, who studies viruses at Christian Medical College at Vellore in southern India, told the Associated Press.

Regardless, many health experts have warned of the variant’s potential to set back progress made in the pandemic. The World Health Organization said the delta variant is "the fastest and fittest" of the known coronavirus strains and warned it will "pick off" those most vulnerable "more efficiently" than previous strains.

"All of these viruses have been lethal in their own regard," Dr. Michael Ryan, executive director of the WHO’s health emergencies program, said during a June 21 news conference. "This virus has the potential to be more lethal because it’s more efficient in the way it transmits between humans and it will eventually find those vulnerable individuals who will become severely ill, have to be hospitalized and potentially die."

Dr. Anthony Fauci, the nation’s top infectious disease expert, also called the delta variant "currently the greatest threat in the U.S. to our attempt to eliminate COVID-19." 

Should I be worried about the delta variant if I’m vaccinated?

Earlier this month, Dr. Anthony Fauci, the director of the U.S. National Institute of Allergy and Infectious Diseases, said all three COVID-19 vaccines approved for emergency use by the FDA — Pfizer, Moderna and Johnson & Johnson — are effective against the delta variant.

"The world is understandably worried about the delta virus variant," Fauci said. "The vaccines indeed are effective against it."

RELATED: Study: J&J COVID-19 vaccine significantly less effective against delta variant

But a study published on July 19 on the medical journal website BioRxiv found that the single-dose Johnson & Johnson vaccine was significantly less effective against the highly transmissible delta and lambda variants compared with two-dose vaccines from Pfizer and Moderna.

Study authors found that while the two-dose vaccines were 94-95% effective in preventing COVID-19, the J&J vaccine had only 66.9% efficacy "in preventing moderate to severe disease."

Study authors said if there continues to be a trend of breakthrough infections accompanied by severe illness caused by the novel coronavirus, the millions of people who have been inoculated with the J&J vaccine may want to consider a booster shot in order to increase protection against variants of concern.

The study authors also said that while there is not currently enough evidence to support the need for a booster, the medical community’s focus should be on primary vaccination efforts in the U.S. and globally.

The new study has not yet been peer-reviewed and is based on laboratory experiments.

"The message that we wanted to give was not that people shouldn’t get the J&J vaccine, but we hope that in the future, it will be boosted with either another dose of J&J or a boost with Pfizer or Moderna," Nathaniel Landau, lead study author, told the New York Times.

Earlier this month, Johnson & Johnson reported that its vaccine is effective against the highly contagious delta variant, even eight months after inoculation.

"We believe that our vaccine offers durable protection against COVID-19 and elicits neutralizing activity against the Delta variant. This adds to the robust body of clinical data supporting our single-shot vaccine’s ability to protect against multiple variants of concern," said Payul Stoffels, chief scientific officer at Johnson & Johnson in a July 1 press release.

The recent findings on the J&J vaccine are consistent with medical observations about the delta coronavirus variant’s resistance to other vaccines.In a study published in The Lancet on June 14, researchers observed patients both vaccinated and unvaccinated who tested positive for COVID-19 between April 1 to June 6, 2021 in Scotland, where the delta variant has become dominant.

Pfizer’s vaccine "offered very good protection: 92% (95% CI 90–93) S gene-negative, 79% (75–82) S gene-positive. Protection associated with ChAdOx1 nCoV-19 (Oxford–AstraZeneca vaccine) was, however, substantial but reduced: 73% (95% CI 66–78) for S gene-negative cases versus 60% (53–66) for those S gene-positive," according to The Lancet.

Authors of the study noted further research needs to be done for more accurate percentage estimates on both vaccines’ effectiveness.

Should I wear a mask if I’m vaccinated?

The U.S. Centers for Disease Control and Prevention changed its mask guidelines Tuesday for people who have been fully vaccinated against COVID-19, citing new information about the ability of the delta variant to spread by those who have been vaccinated.

The CDC is now recommending that vaccinated people wear masks indoors again in parts of the U.S. where the coronavirus is surging and that everyone in K-12 schools wear masks, regardless of vaccination status. 

CDC Director Dr. Rochelle Walensky said the high transmissibility of the delta variant is behind the agency’s change in guidelines.

RELATED: New CDC mask guidelines: Vaccinated should mask indoors, as should K-12 schools

"Unlike the alpha variant that we had back in May, where we didn’t believe that if you were vaccinated you could transmit further - this is different now with the delta variant. And we’re seeing that now, infection is possible if you (have been vaccinated and) are a rare breakthrough infection, that you can transmit further, which is the reason for the change," she said.

The new guidelines Tuesday come as a growing number of cities and towns have restored their own indoor masking rules.

Earlier this month, Los Angeles County re-implemented its indoor mask mandate, regardless of vaccination status. St. Louis, Savannah, Georgia, and Provincetown, Massachusetts, are among the places that followed suit.

In May, the CDC issued guidance that said vaccinated people could go without masks in most settings. They are still required in places like airplanes, buses and trains.

RELATED: Fauci: CDC ‘carefully looking’ at mask guidance for schools

This story was reported from Los Angeles. Jordan Smith, Megan Ziegler, Kelly Hayes and The Associated Press contributed.