Because of the Delta strain, Los Angeles County is once again requiring masks to be worn indoors regardless of vaccination status.
If experts predicted which variant of coronavirus would take over the globe, the Delta would not have been their first devination. However, the high contagious version became the prevalent strain of this virus since its first appearance in India in December 2020, accounting for more than 90 per cent of new COVID cases in the United States.
The introduction of Delta has led to a number of countries restoring restrictions on travel and masks that were removed by increasing vaccination rates. Although Delta vaccines seem to be most effective, the amount of doses in the vaccinated persons increases the probability of causing “breakthrough infections.” And if it causes more serious diseases than the previously circulating strains is yet uncertain.
However, it is obvious that Delta has a considerable advantage over earlier strains. “The rate of growth in this pandemic is unlike any other in history,” says Vaughn Cooper, a University of Pittsburgh evolutionary researcher. Now, he and others are trying to figure out why this particular variety, which has several mutations, was so effective.
The rapid spread of the virus is partly due to a rapid replication rate. A recent study found that persons with Delta infections had almost 1000 times more viruses in their body – called viral load – on average than those infected with the original strain, allowing them to infect many more people quickly. The benefit of the variation seems to be a mix of spike protein mutations: the part of the new coronavirus that connects cell surfaces with ACE2 receptors that allow the virus to infect them.
Scientists have also questioned whether Delta is able to avoid the human immune system in addition to its heightened transmissibility. There is no mutation termed E484K, which helps to prevent certain other variations from being neutralised by antibodies. But laboratory tests reveal that a Delta mutation termed L452R performs the same purpose even better.
The epidemiologist Nathan Grubaugh from the Yale School of Public Health in New Haven, Conn., in a recent preprint study that has not yet been published in a journal, tested how 18 different variants of 40 fully vaccinated healthcare workers react to serum taken from plasma—the liquid blood component. They found that the antibodies of the participants could neutralise the Alpha variants and Delta rather well. However, they were less effective against E484K mutations, such as beta and gamma, that were initially found in South Africa and Brazil, respectively.
The results were surprising, as Delta was far more effective than Beta or Gamma in spreading. While the L452R mutation alone increased immune evasion, the actual Delta virus is not so good, and the precise combination of Delta mutations has given the virus a distinctive role. “It’s difficult to find out, even when we try to simplify things, which combinations are to become the next ‘it’ virus,” adds Grubaugh. He adds that the success of Delta indicates that immune evasion might not give the virus an evolutionary benefit as strong as transmissibility in uninfected humans at least.
This may not be true in every population, though. For instance, the Gamma variation spreads fast in Brazil, but relatively little across the rest of the world. Some scientists suggest that COVID rates in Brazil could have been higher than those of most countries, thus the majority of people were already in a position to develop a powerful immune reaction by the time Gamma emerged. Avoiding the immune system would have been favourable for the virus in this group.
Mehul Suthar, a virologist at Emory University in Atlanta, believes the number of mutations that a virus can acquire on a spike protein can be limited before it can bind to the ACE2 receptor. “This tug-of-war will always take place between the virus’ ability to transmit, disseminate, and multiply and its ability to escape antibody reactions,” adds he.
The Kappa variety, which evolved in India about the same time as Delta, has most changes in Delta and a mutation identical to E484K, Suthar points out. Kappa, however, has not expanded worldwide, demonstrating that these changes can interact to reduce the evolution of the virus. “It was a little surprising that the Delta variation finally took off,” adds Suthar. “It felt comfortable, actually.”
Grubaugh believes it is unlikely that there will be major new spike protein mutations. Instead, a mixture of the “best” mutations will make the fittest virus spread to most individuals. But it’s difficult to forecast what it’ll be. “I think we have a very good grip on what mutations we should search for, but if we look at them, that doesn’t always mean that we know exactly how they are,” he says.