Two weeks ago, the CDC labeled the Delta variant as a “variant of concern.” Formerly known as B.1.617.2, or as one of two variants first identified in India, Delta was thought to be partially responsible for the rapid rise of cases in that country, and was known to share some of the features that made the earlier Alpha variant more contagious. But as more information comes in, “concern” doesn’t seem to express the level of anxiety that might be attached to this variant—which is rapidly becoming dominant in the United States and around the world. That’s because the Delta variant seems more than capable of reinfecting those people who have been infected by previous forms of the virus behind COVID-19. And it’s much more vaccine evasive than anything tested before.

And now there’s an “upgrade” to that variant in the form of B.1.617.2.1. This variant, also known as “Delta Plus” was also first detected in India earlier this year. This new variant has been seen in numerous countries, including the United States, and may have features that make it still more contagious and vaccine-evasive than the widespread form of Delta.

The original Delta variant is bad enough. Bad enough that a Nature preprint from scientists in India and the U.K. shows that this variant is turning previously rare COVID-19 reinfections into a regular occurrence. The vaccine is also 8-9 times less responsive to antibodies produced by vaccines. That doesn’t mean that vaccines are useless. In fact, vaccines may be the only thing that stops Delta at this point, as the response from vaccines is still well above that generated by the immune systems of people who have previously been infected by the Alpha or other variants of SARS-CoV-2 virus. 

Delta isn’t just spreading quickly because it is more contagious. It’s spreading quickly because it is blowing past the immune systems of those who have previously been infected.

As Cambridge microbiology professor Dr. Ravi Gupta explains in an extensive thread on the results, modeling of the huge recent wave of cases in India suggests that many of those who got sick with the Delta variant were previously infected. That includes those who had asymptomatic or mild cases of COVID-19 and thought that this previous exposure would generate immunity. 

But as both the in vitro research and modeling shows, Delta was not just 10-40% more transmissible than even the already highly contagious Alpha, it was also capable of evading neutralizing antibodies in 20-55% of those who had previously been infected by a non-Delta variant. Delta spreads more readily, and those who had a previous case—especially those where a mild case may represent a low level of immune response—are absolutely not protected from this variant. This is a very bad combination.

Not only does this version of the virus seem to generate more spike proteins, giving it a easier access to human cells, it also replicates in greater numbers, resulting in a higher viral load in the respiratory system of those infected. That gives Delta a boost in moving to the next victim, and also makes it more likely to cause severe disease. Increased transmissibility plus immunity evasion makes Delta a threat to even places that have been ravaged by past waves of COVID-19, and makes it a serious threat to those who claim they don’t need to be vaccinated because they already had the disease.

Vaccine response of the Delta variant

The Delta variant is also infecting people who have been vaccinated at a rate well above any previous variant. Across India, the Delta variant “dominates vaccine-breakthrough infections.” However, these cases are still extremely rare. In vitro testing of the Delta variant against antibodies generated by vaccines from AstraZeneca and Pfizer/BioNTech showed that the response was 8.4 to 9 times less than that seen with the original “wild type.” To some extent, that should be expected. Both vaccines spike proteins created from the original genetic sequence of SARS-CoV-2 which was provided in January of 2020. Every change to that protein that has happened since then has reduced the impact of those vaccines. However, “reduced” does not mean “negated.” Not even close. The response generated by vaccines is still two orders of magnitude greater than that seen when looking at samples taken from those previous infected. Vaccine evasion by Delta happens. However, the vaccines are still effective at blocking almost all cases, and when infections do occur, vaccines are effective in reducing the severity of illness.

As the Associated Press reported on Thursday, almost all COVID-19 deaths in the United States are now among the unvaccinated. And “almost all” seems like an understatement. Because the percentage of COVID-19 deaths occurring among vaccinated patients was %0.8 over the month of May. That means 99.2% of all deaths occurred in the 46% of the population that is unvaccinated. This remains true, even as Delta forms an ever higher percentage of cases.

It also means, as CDC Director Dr. Rochelle Walensky said this week, “nearly every death, especially among adults, due to COVID-19, is, at this point, entirely preventable.” Over 99% are the pointless expression of continued high vaccine disdain among Republicans. 

Still, vaccinated people are getting infected. The international study looked at 100 health care workers who had become infected after being vaccinated. As other studies have shown, these vaccinated workers didn’t get as sick as others who were infected by the Delta variant. However, those infected with Delta still carried high viral loads. So even the vaccinated-infected could easily transmit the virus.

And that’s being seen in the real world. As The Wall Street Journal reports, an outbreak of Delta variant is spreading across Israel, with about half of those infected being fully vaccinated. However, this doesn’t meant that the Delta variant can completely bypass vaccination. The vaccination rate in Israel, while high, is still less than 65%. That’s below the projected level needed to halt community spread, even of the original version of SARS-CoV-2. The ability of the Delta variant to not just infect the vaccinated, but be passed along, changes the calculus. It’s unlikely that the R0 of the virus is such that it can sustain community spread in a community where everyone is vaccinated—the rate of infection is too low. However, the level of vaccination needed to evade Delta is going to be considerably higher.

This outbreak in Israel is, at the moment at least, not a nationwide wave, but more of a cluster. It’s likely that this outbreak will not be sustained. And, thanks to vaccines, it’s not generated a large number of severe cases. But it shows that an insufficiently vaccinated population is susceptible to the Delta variant, even when the national average is high. This particular outbreak started among a group that hasn’t been vaccinated—school children under the age of 16—and spread to their families. It shows how hard it will be to close this thing down so long as significant cohorts remain unvaccinated. Israel responded quickly by reinstituting mask requirements and dropping plans to reopen the country to international tourism in the next month.

In the United States, where the percentage of people vaccinated is still less than 20% in some counties, the variant could easily sustain community spread. And while the vaccinated may be protected from serious disease, the unvaccinated are not. Hospitals in some areas are already seeing increased loads of younger, sicker patients infected by the Delta variant.

The increased threat of community spread also means that some areas need to rethink relaxed guidelines and attempts to “get back to normal.” This is already impacting facilities like nursing homes in Colorado, where the Delta variant is threatening seniors whose immune systems don’t generate as strong a response to the vaccine.

Whether any of these things get worse with Delta Plus isn’t clear. And there is already many other variants that branch from the Delta variant. Any one of which may be more vaccine evasive.

The faster everyone gets a vaccine that reduces their chance of being infected, which in turn reduces their chance of spreading the disease, the more likely the world is to slow the parade of variants before a genuinely vaccine-evasive version enters rapid spread. However, everyone who got a vaccine in the last year should be expecting a booster in the next year, because the only way to be sure of keeping up vaccine resistance, is to try and keep up with the changing mix of viral proteins.