What is the delta variant? The fast-spreading Covid-19 mutation in the US, explained.

The fast-spreading delta variant of the coronavirus is eroding some of the world’s precious progress against the Covid-19 pandemic. It’s likely the most transmissible variant of the SARS-CoV-2 virus identified to date, appears to cause more severe illness than others, and has already landed in at least 85 countries.

While health experts are worried, much of their advice hasn’t changed. The strategies that have contributed to progress so far — masks, social distancing, and especially vaccines — on the whole remain effective. But these tools work best when everyone is willing to use them, and those who don’t are at the greatest risk.

Even countries that have done a good job of vaccinating people are starting to hit the limits of people willing to get their shots, leaving smaller groups still vulnerable to the disease. In places like the US, this pattern has effectively created two distinct pandemics, with vaccinated people starting to return to normal while those who are unvaccinated make up almost all new hospitalizations and deaths from Covid-19.

Under Centers for Disease Control and Prevention guidelines, fully vaccinated Americans do not need to wear masks or maintain social distancing. But World Health Organization officials made a different recommendation on June 25, encouraging even vaccinated individuals to wear masks in hopes of preventing the spread of variants like delta. Los Angeles County officials suggested similar precautions for indoor settings on June 28, referencing the delta variant.

In the coming months, even as the coronavirus continues to mutate, the human element — the willingness to get vaccinated, adjust behavior, and tolerate restrictions — will be critical in containing the disease. “As has been true for the past year and a half, human behavior is far more important in shaping the course of the pandemic than any variant,” wrote virologists Amy Rosenfeld and Vincent Racaniello in the New York Times.

But after 16 months of restrictions, many people may be reluctant to modify their routines to counter evolving risks from variants. This urge to return to normal, if mismanaged, could end up drawing out the pandemic further and creating opportunities for dangerous variants to make inroads.

The delta variant is poised to dominate new Covid-19 cases

In the US, the delta variant of SARS-CoV-2 currently accounts for 20 percent of new cases and is on track to become the dominant variant in the US. “The delta variant is currently the greatest threat in the US to our attempt to eliminate Covid-19,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, during a briefing in June.

In Israel, the variant is responsible for up to 70 percent of new cases, prompting the country to reinstate an indoor mask mandate. Health officials in Israel reported that about half of the new cases were in adults who had been fully vaccinated. In the United Kingdom (which has vaccinated a large portion of its population) and Uganda (which has not), delta comprises almost all new infections. The same is true in Australia and parts of Asia, which had reduced their rates of community spread to enviably low levels but are now imposing new lockdowns in some cities to control the spread of the variant.

Delta itself is continuing to change. Health authorities reported last week that a sub-variant with additional mutations, delta plus, has caused at least 50 infections in India, where the delta variant was first detected last year. Delta plus has already been reported in 11 countries.

Vaccines — the most effective tool for countering Covid-19 — remain potent for the most part at preventing severe illness and death from these mutant forms of the virus. A study published in The Lancet looking at Scotland found the vaccine developed by AstraZeneca and the University of Oxford had 60 percent efficacy in preventing illness from the delta variant, while the Pfizer/BioNTech vaccine had 79 percent efficacy. Both vaccines, however, were effective at preventing hospitalizations from the delta variant. Moderna reported early results showing that its Covid-19 vaccine was able to generate an immune response to delta, albeit with a “modest reduction” compared to the original virus.

It’s less clear how well the two Chinese vaccines, developed by Sinopharm and Sinovac, fare against delta. These vaccines are widely used in Asia, Africa, and South America. There is emerging evidence that they may be less effective than some of the others on the market against SARS-CoV-2 in general, and delta in particular.

But even the best vaccines are not impregnable. Some infections still occur among those who have been vaccinated, and immunized people may be able to spread the virus. Still, with rare exceptions, these breakthrough infections usually lead to no symptoms or mild symptoms, and the rate of transmission by vaccinated people is drastically lower.

In order to yield their maximum public health benefits, vaccines have to be administered to as many people as possible, getting to the point where the virus can no longer spread easily from person to person. However, there are cavernous gaps in Covid-19 vaccination rates around the world, creating ample space for the virus to run rampant.

Now the spread of the delta variant is amplifying the divisions between vaccinated and unvaccinated people. “If you are vaccinated, this should not change your thinking,” said John Moore, a virologist and vaccine researcher at Weill Cornell Medical College. “If you’re not vaccinated, you’re at increased risk of getting infected.”

Why the delta variant appears so dangerous

The delta variant of SARS-CoV-2, initially known as B.1.617, is one of five variants of concern identified by the Centers for Disease Control and Prevention. A variant of concern is one that spreads more readily, causes more severe disease, or reduces protection from vaccines or previous infections. Delta checks all three boxes.

Like all viruses, SARS-CoV-2 accumulates mutations as it spreads through a population and makes errors in copying its genetic code. The majority of those mistakes are harmful to the virus or have no net effect, but occasionally a mutation will arise that gives the virus an advantage. Health officials then classify strains of the virus with distinct groupings of mutations as variants.

Delta contains at least 23 mutations compared to the original strain of SARS-CoV-2, but there are several mutations that confer particularly alarming traits. Among them, delta has mutations in the spike protein of the virus, the part the virus uses to attach to human cells to infect them. Some of these mutations increase the affinity of the spike for a receptor on human cells, allowing the virus to reproduce more efficiently.

That could allow the variant to make more copies of itself compared to the original version of the virus, wreaking more havoc throughout the body. And more viral particles in the body could also mean the infected person is shedding more virus, increasing the likelihood of the infection being passed to someone else. The delta variant is about 60 percent more transmissible than the alpha variant, and the alpha variant was already up to 60 percent more transmissible than the original version of the virus.

“The most troubling aspect is its transmissibility,” said Moore.

The spike protein of SARS-CoV-2 is also the main target for the immune system when the human body fights an infection. White blood cells known as B cells make proteins called antibodies that stick to the virus, often at the spike protein. These antibodies then flag the virus for removal by other white blood cells, or prevent the virus from causing an infection. But antibodies are very particular about binding to specific parts of the virus, and if those parts mutate, then the antibodies become much less effective.

Scientists have found that delta has mutations in exactly those regions, making it better able to evade antibodies produced to counter past versions of SARS-CoV-2. That increases the odds of a Covid-19 survivor getting reinfected with delta. It also means that delta has a greater chance of causing a breakthrough infection in someone who has been vaccinated against Covid-19, since many of the vaccines train the immune system with an older version of the spike protein.

There’s evidence as well that delta can cause more severe outcomes from Covid-19 compared to the original versions of the virus. One study looking at 5.4 million people in Scotland found that infections with delta are more than twice as likely to lead to hospitalizations as infections with the alpha variant, previously known as B.1.1.7. However, the death rate from delta appears to be comparable to other variants in several countries, blunted in part by vaccines and the increased capacity of hospitals at this stage in the pandemic. For instance, cases in the United Kingdom have risen back to the levels they reached in January, but the death rate is vastly lower now than it was then.

There have been some reports that younger people are making up a larger proportion of new delta Covid-19 cases, but that’s due in part to young adults, teenagers, and children being the last groups allotted to receive vaccines, as well as a lower uptake of vaccines in these groups even when offered.

By far the biggest threat from delta is for people who have not been vaccinated or had a previous infection. Remember that the original version of SARS-CoV-2 could spread easily between people, oftentimes unwittingly. And the virus caused a disease that led to millions of deaths around the world, as well as lasting ailments and untold economic and social damage.

So a mutated version of the original virus that is more transmissible, leads to more severe outcomes, and is harder to protect against stands to be devastating for people who have no protection. And, indeed, delta infections are occurring largely in unvaccinated people, accounting for a growing share of hospitalizations and deaths from Covid-19 around the world.

The looming threat now is that the more delta spreads, the more likely it is to mutate further in dangerous ways. “The scary scenario is that this is not the last or the most harmful variant,” said Maureen Miller, an infectious disease epidemiologist and medical anthropologist at Columbia University. And when variants cause new lockdowns, the restrictions affect everyone — not just the unvaccinated people who are most at risk.

The old formula for containing Covid-19 still works against delta

Despite the risks from delta, the overall public health strategy used throughout the pandemic remains effective. Many of the places that have suffered the worst effects of the delta variant, like India, also struggled to maintain pandemic control measures, so it’s hard to attribute the spikes in cases to the variant alone.

Getting vaccinated against Covid-19 is the strongest form of protection. While some vaccines may be less effective at blocking infections from delta and other variants, many of the vaccines were so good at preventing disease to begin with that even a diminished level of efficacy is still robust.

So far, the evidence shows that the vaccines approved in the US — from Moderna, Pfizer/BioNTech, and Johnson & Johnson — still prevent almost all hospitalizations and deaths. Some vaccinated individuals in the US have experienced more severe disease, and several have died of Covid-19 after vaccination, but the majority of breakthrough infections resulted in mild symptoms. Some researchers have suggested that recipients of the one-dose Johnson & Johnson vaccine may need to get a booster dose to better protect against delta.

However, other countries are facing complications from the vaccines that they’ve approved, with some vaccines shown to be less protective against SARS-CoV-2 in general and others failing to provide adequate protection against variants. Countries like Mongolia and Seychelles, an island nation in the Indian Ocean, relied on vaccines from China and are still seeing high levels of spread of Covid-19. It’s hard to distinguish whether these cases are due to a less effective vaccine, a more slippery variant, less compliance with pandemic controls, or a mix of the three — but taken together, it’s an example of diverging experiences of the pandemic across the world.

People who survived a bout with a previous version of Covid-19 are also more likely than not to shrug off an infection with the delta variant, although the level of protection from a past infection is less than that of a vaccine. “We should expect more reinfections with delta,” said Moore. “That’s why previously infected people should also get vaccinated.”

For people who have yet to be vaccinated against Covid-19, practicing good hygiene, wearing face masks in enclosed environments, and social distancing can still reduce the risk of infection.

Many of the treatments for Covid-19 remain about as effective against Covid-19 cases caused by delta, though monoclonal antibodies are key exceptions. These drugs use antibodies targeted at a single part of the virus, and if that part mutates, they are rendered ineffective. The Food and Drug Administration has already revoked an emergency authorization for a monoclonal antibody drug that failed to mitigate infections caused by SARS-CoV-2 variants. On the other hand, monoclonal antibodies are expensive drugs that require transfusion, so they aren’t used that widely, and losing some of them is unlikely to make a major difference in overall patterns of the disease.

Testing to identify potential asymptomatic spreaders and directing them to isolate should remain a pillar of the public health response to Covid-19. Sequencing the genomes of SARS-CoV-2 samples in circulation is another critical step in identifying and staying ahead of new mutations. It allows scientists to find variants that can escape the protection offered by vaccines. Without widespread genome sequencing, variants can spread undetected.

Ongoing and reimposed pandemic restrictions are sparking frustration

A major problem now is that pandemic fatigue is setting in, and countries like the US that have done well in vaccinating their populations risk becoming complacent. Testing rates are declining. Vaccination rates are hitting a plateau. And only a tiny fraction of virus samples are being sequenced.

“That is a public health concern, and I’m nervous that in the United States we’re not keeping as close track of breakthrough infections that don’t lead to hospitalizations and deaths as we should be,” said Miller.

Many public health tactics are getting harder to deploy. Already, in parts of the US, people have rallied against mask mandates and vaccine requirements for hospital workers. Protests against new lockdown restrictions have also erupted in the UK, while countries like India that have only vaccinated a tiny fraction of their populations saw protests against lockdowns even while suffering a devastating outbreak.

At the same time, economies are struggling, and many people are desperate to return to work. Some are past the limits of their financial means. So the political will to reimpose restrictions if cases rise is much weaker now, and people could be less inclined to comply.

“Changes in people’s activities contribute to the rise of infections — such as travel, failure to mask and to adhere to physical distancing policies, and most important right now, insufficient vaccination — and these are often not considered in public discussion of variants,” wrote Rosenfeld and Racaniello.

These social factors also make it harder to disentangle the inherent dangers of a variant like delta from public health failures. The virus itself may be more dangerous, but it’s difficult to know exactly how dangerous when people are unable or unwilling to take steps to slow it.

The delta variant is creating a divided pandemic in the US and around the world

While people who have been immunized against Covid-19 are still shielded, those who aren’t now face a more dangerous virus along with a decreased tolerance for pandemic restrictions. In the US, areas with low vaccination rates overlap with areas where other containment strategies have met resistance.

“It’s probably not surprising that the places where [mask-wearing and social distancing] were least accepted are places where vaccines are low,” Miller said.

The tragedy is that everyone who dies or falls ill from Covid-19 from here on out will do so at a point where there are multiple highly effective vaccines, which drastically reduce the most harmful aspects of the disease. And while close to half of US adults have been vaccinated against Covid-19 and many places have undone all their pandemic restrictions, there are still parts of the country where cases are rising.

“Cases have begun to rise more rapidly in communities with lower vaccination rates,” noted journalist David Leonhardt at the New York Times. Rural areas in states like Arkansas, Missouri, Nevada, and Utah are a case in point.

One hospital system in southwest Missouri reported a 225 percent increase in hospitalizations from Covid-19 since the beginning of June. Only 39 percent of adults in Missouri are fully vaccinated as of June 29, and the delta variant accounts for more than a quarter of new cases in the state, the largest share of any US state.

So the topline vaccination rate across the country obscures the fact that there are pockets that are far more vulnerable to the delta variant. With temperatures rising this summer, people seeking cooler spaces indoors could end up facing an even higher risk of exposure. Not only will this add to the public health burden, it will also increase the opportunities for further mutations in the virus.

The same expanding divide exists on a global scale, with many wealthier countries lifting restrictions and returning nearly to normal, while countries with fewer resources are coping with an even more dangerous virus.

In some countries, vaccination has barely begun. Countries like Chad, Benin, Syria, and Yemen have immunized just a fraction of a percent of their populations. Others, like Uganda and Zambia, are now dealing with their second or third wave of cases and seeing hospitals overwhelmed. Many are counting on vaccine donations from well-stocked nations such as the US, or are trying to develop their production systems.

While restaurants, offices, schools, and theaters are reopening in some parts of the world, it’s important to remember that Covid-19 remains a global threat, and it’s still far from over. Less than a quarter of the world’s population has received a single dose of any Covid-19 vaccine to date, which means the majority of people continue to face a deadly disease with little to no protection. Vaccination rates need to speed up drastically to stall an even larger humanitarian toll and contain the virus. Otherwise Covid-19 will continue destroying lives and livelihoods while transforming in more dangerous ways.

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