This raises many questions, and the three most important are: Will it become more virulent — causing more serious disease and death? Will the virus escape the protection natural immunity and vaccines now afford? And, if the answer to either of the first two questions is yes, how can we respond?
Right now, the best we can do is make educated guesses. There’s no solid information yet on delta’s virulence, although it seems more dangerous. It wreaked havoc in India, but it’s difficult to know how much of the death toll can be attributed to increased virulence and how much to an overwhelmed health-care system. Anecdotal accounts here also speak to increased virulence, including in younger adults. We also know that delta produces about 1,200 times the viral load of the original virus — and viral load correlates with severity and death. That fact is not comforting.
Neither is history. All five influenza pandemics we have details about developed more virulent variants before settling down. The pandemic beginning in 1889 was more than twice as deadly in Britain in the second year as in the first, and in many countries the third year was deadlier still.
In 1918′s epidemic, the first wave was both mild — the British Grand Fleet suffered 10,313 first wave cases but only four deaths — and not very transmissible. A variant caused an explosive second wave.
The 1957 influenza pandemic led to a significant increase in deaths, but in 1960, after both a vaccine was developed and many people supposedly had immunity from prior infection, a variant caused peak mortality to exceed pandemic levels.
In 1968, the United States saw the most deaths in the first year, but in Europe — again after a vaccine and naturally acquired immunity were in play — the second year was deadlier.
During the 2009 influenza pandemic, variants emerged that caused breakthrough infections and increased viral loads and deaths in the United States, and studies found “greater burden of severe illness in the year after the pandemic” outside the United States as well.
As a general rule, viruses do eventually become less dangerous as they adapt to new hosts and as immune systems respond better. That should happen here eventually. But whether or not delta has increased in virulence, another still more dangerous variant may surface.
That makes the next question even more important: Will covid-19, in some form, escape immune protection? The answer is: probably.
Unless its opportunity to mutate is cut off by stopping its spread — an impossibility with billions worldwide unprotected by vaccine — eventually a variant will likely emerge that evades current vaccines and natural infection. Studies of coronaviruses that cause the common cold demonstrate that mutations over time cause the ability of antibodies to neutralize those viruses to decline.
Laboratory studies also show a decline in neutralizing antibodies against covid-19 after a year, although antibodies remain effective enough to provide protection, and other elements of the immune system remain robust. Outside the laboratory, the gamma variant seemed to almost entirely escape natural immunity from prior infection, and vaccine effectiveness has declined marginally against several variants.
What does this tell us about plotting our next steps?
First, even if the virus does escape vaccine protection, it will happen very gradually — and does not necessarily mean disaster. We should have time to adjust vaccines, and so far, Moderna, Pfizer, Johnson & Johnson and Novavax (which will likely get emergency authorization from the Food and Drug Administration soon) all protect against all known variants. But even if their efficacy declines significantly, they will still save huge numbers of lives. Influenza vaccines are routinely only 30 to 50 percent effective in preventing illness but still exceed 80 percent efficacy in preventing ICU admission.
Second, whether we need booster shots with existing vaccines or not, we will eventually require updated vaccines targeted against the latest variant, just as we do each year for influenza. Even if delta is the worst variant we see, the virus will continue to mutate. As with influenza, the goal is to develop a vaccine that protects against all variants.
In the best case, delta will be the most dangerous variant to emerge, our immune systems will learn to respond, trained by vaccines or infection, and we will achieve the Holy Grail of a kind of herd immunity. This will still not mean actual immunity; it will simply mean the disease becomes endemic, and infections and deaths will continue to occur, but in much reduced numbers. Meanwhile, for at least a year — and likely longer — the vast majority of the world will continue to see significant mortality and social and economic disruption.
The pandemic isn’t over, not even for those who are vaccinated. The virus remains the boss. Covid-19 may still surprise us, and if it does, at this still middle-aged stage of the pandemic, it’s more likely to be an unpleasant surprise.