How Worried Should Vaccinated People Be About the Delta Variant?

A bottle of the Moderna covid-19 vaccine.

A bottle of the Moderna covid-19 vaccine.
Photo: Joe Raedle (Getty Images)

The arrival of the Delta variant—the recently emerged strain of the coronavirus thought to be the most transmissible yet—in the U.S. is understandably unnerving scientists and the public alike. But how worried should you exactly be about Delta, especially if you’re fully vaccinated already?

Delta, also known as B.1.617.2, was first discovered in India last October. Evidence, largely from the UK and India, suggests that it’s noticeably more likely to spread from one person to others than the Alpha variant (also known as B.1.1.7), the strain first found in the UK that was already more transmissible than the original lineages of the virus that first spread around the world. Estimates vary, but Delta may be around 40% more transmissible than Alpha and up to twice as transmissible as the classic coronavirus.

Alpha quickly established itself as the dominant strain in many countries, including the U.S., earlier this year, and now Delta has done the same in at least the UK and India. In the U.S., about 20% of new cases are now thought to be caused by Delta, and within the next month or so, it’s almost certain to be the new dominant strain.

At this point, it’s not clear if Delta is simply more transmissible than past strains or if it’s also more likely to cause illness and death in an individual person. But even if it were only more contagious, scientists are rightly worried about its potential to spread faster and wider than past outbreaks of the pandemic. It’s already thought to have fueled the largest and deadliest peak in India, which has reported almost 400,000 deaths total (in truth, likely a major underestimate). It also seems to be behind the renewed waves of illness ongoing in Africa.

Most of these countries still have low to non-existent vaccination rates. But Delta has also been making a splash in highly vaccinated countries like the UK and Israel. In the UK, Delta has clearly been responsible for a surge of reported cases in the last month or so, and in Israel, an increase of cases linked to Delta prompted the government to reinstate rules on indoor mask-wearing last week, days after it had lifted them. Over the weekend, the World Health Organization similarly called for fully vaccinated people to continue wearing masks in light of Delta.

All of this is to say that Delta is the real deal—a more dangerous version of an already highly contagious disease that has killed millions in the past year. It’s not accurate to say that vaccinated people have absolutely nothing to fear from Delta. But, thankfully, the risk appears to remain objectively very low.

The UK’s surveillance of the virus, likely some of the best in the world today, has not only charted Delta’s emergence in the country, it’s also provided clear data on the risk it poses to people fully vaccinated with the Pfizer/BioNTech mRNA vaccine or the AstraZeneca vaccine. This data has suggested that the first dose of either vaccine doesn’t provide as much protection against illness from Delta as it did against other strains—a sign that the variant contains some mutations that make it look a bit different to an immune system trained against the classic version of the virus. But this protection is only slightly lowered in people fully vaccinated with two doses. And most importantly, both vaccines continue to offer highly effective protection against hospitalizations and deaths from Delta—upwards of 90% effectiveness.

This is borne out by national-level data too—even as cases have significantly climbed back up in recent weeks, hospitalizations have only slightly increased, while deaths haven’t at all. In Israel, which has the highest fully vaccinated rate in the world at almost 60%, cases have jumped a bit, but not deaths and hospitalizations.

A more open question concerns the Johnson & Johnson vaccine, which only 12 million Americans have taken. Right now, there’s simply no direct data on how the efficacy may change with Delta. But other evidence from past variants like Alpha suggests that Delta should, at worst, modestly lower your level of protection from any illness, but protection from more serious illness should remain high with the J&J vaccine. That said, some experts are speculating that a booster shot, likely with a mRNA vaccine, could be valuable for those with only the J&J shot.

Delta, like Alpha before it, does represent a more fit upgrade to the original strain of this coronavirus (fit = more likely to replicate in our cells). But novelty has always been the greatest strength of SARS-CoV-2, and once someone has had exposure to it, through vaccination or past infection, the body’s immune system has its own bag of tricks to counteract the now-familiar germ. Like viruses, for instance, our immune cells can produce slightly mutated versions of antibodies that try to anticipate how it’s likely to evolve. This adaptivity means that it’s not easy for a virus to completely fool a functioning immune system that’s seen some past form of it.

Sure, the coronavirus may eventually evolve enough to substantially evade our immunity. But Delta doesn’t seem to have accomplished that goal, based on all the information to date (researchers in India have recently warned about Delta-plus, a version said to carry an added mutation that may aid in immunity-escape, but there’s no conclusive evidence of that either right now). New research just published today, in fact, suggests that vaccine-provided immunity from the mRNA vaccines has the potential to last for at least a few more years.

Unfortunately, even in the U.S., there are still areas of the country where vaccination rates are far lower than the national level (63% of eligible people have at least one dose). Immunocompromised people who have been vaccinated but don’t create as robust a response as others may also be at higher risk for illness, and there are millions of kids under 12 who simply don’t have access to vaccination right now.

Even when you take into account that many unvaccinated people in America may have already had an infection, though, there remains the possibility that Delta will lead to regional spikes of the pandemic here. Like before, these spikes will be driven by people with no past exposure to any version of the coronavirus at all. But unlike past peaks, most every case of serious illness and death is now easily preventable through vaccination, even in the face of worrying variants like Delta.