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It’s clear: the coronavirus is becoming more and more transmissible. Since the virus first appeared in China, it has been get mutations which help it to spread more easily among humans. The Alpha variant, first detected in the UK last year, is 50 percent more transmissible that the original version, and now the Delta variant, first detected in India, is at least 40 percent more transmissible than Alpha.
What is less certain, however, is how the increased transmissibility of the virus will affect the pandemic in the United States. The arrival of Alpha has raised concerns about a new push in the spring, but one never came. The proportion of Alpha cases kept going up, but the total number of cases continued to decline. People got vaccinated. Alpha has become dominant in the United States. The cases have declined further. The virus had become more biologically transmissible, but it was not spreading to more people.
There was one notable and puzzling exception: In April, Michigan saw a spike in cases that experts said were indeed Alpha fueled. The fact that the variant had such different consequences for Michigan than for the rest of the country shows how difficult it is to make predictions. Alpha has little effect on vaccines, but fears variants that slightly erode vaccine protection, Beta and Gamma, have also calmed down; neither causes significant spikes in cases among the vaccinated. “If there’s a time when we need to be humble, it’s around this issue,” says Michael Osterholm, infectious disease epidemiologist at the University of Minnesota.
Delta has garnered so much attention because it has the most disturbing collection of traits to date:but not those who had two), and can even cause more serious illness. All of this is enough to be a warning, especially since Delta is now responsible for 10 percent of American cases and rising. But as with Alpha, which was also believed to be more serious, the variant’s behavior in the real world will depend on more than its biology. It will also depend on how we, the hosts of the virus, choose to behave, how many more people we vaccinate, and to some extent how lucky we are.
All of these factors are likely to have played a role in the spring peak associated with Alpha Michigan. According to cell phone mobility data from that period, residents of the state had returned to almost pre-pandemic levels of movement, says Emily Martin, an epidemiologist at the University of Michigan. The Alpha variant also arrived in Michigan relatively early and ended up in groups of young people who were not yet eligible for vaccination. “It was kind of bad timing,” Martin told me. If Alpha had arrived a little later, or the vaccines a little earlier, Michigan might have been more like the rest of the country, where the vaccination may have lessened Alpha’s impact. In the race between variants and vaccines elsewhere in the United States, vaccines have won.
Two concepts about viral spread help explain why timing and chance make such a difference. First, the coronavirus is spreading exponentially, which means that even a slight delay in mitigation efforts can lead to drastically different results. Second, the spread of the virus is what epidemiologists call “overdispersedWhich means that the majority of patients infect no one other than a small handful could infect dozens of people. In other words, most sparks of infection don’t ignite. But sometimes a single infection can cause an early super-spreading event, which ends up triggering a major outbreak. “Looking from state to state, it might be like, ‘Well, why is this state doing well compared to this state? “Sometimes it’s just luck,” says Adam Lauring, a virologist at the University of Michigan.
To predict the behavior of the variants, much of the world turned to the United Kingdom, where a excellent and comprehensive genomic monitoring program followed the rise of Alpha and now Delta. Alpha accounted for 98% of all COVID-19 cases in the UK at the peak of this variant in March; Delta has since taken over, representing almost all new cases. It is too early to say whether the United States will follow the same path. Alpha was responsible for anywhere 38 to 86 percent of all new cases in the United States last month, according to the state. Nathan Grubaugh, an epidemiologist at Yale, says this fact suggests the limits of the comparison of the two countries. “The US is much more heterogeneous than the UK,” he told me, with a greater diversity of viruses and greater geographic differences in vaccine uptake. As far as Delta is concerned, he said, “that means some places will be hit harder.” And most likely, these places will be the ones where fewer people have been vaccinated.
Experts agree that vaccines are the best way to stop Delta. UK data suggests that one dose of Pfizer vaccine provides only 34 percent protection against the variant, while two doses provide 88 percent. Large swathes of the United States, however, still struggle to get people to take doses at all. A recent Washington post Analysis found over 100 counties where less than 20 percent of the population had been vaccinated. “Whatever loopholes we have in our community immunization program, this is what Delta is going to exploit,” said Martin.
The United States has at least an adequate supply of vaccines to fill these gaps; the challenge is social. In countries without sufficient doses to stop it, Delta portends many more problems. In some ways, it’s a luxury to say that the eventual effects of Delta in the United States are unpredictable.
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