Eighteen months ago a novel virus that humans had no immune defenses against began bringing the world to its knees.

Powerful, protective vaccines were quickly developed, raising hopes for a return to normal. But many people in wealthy countries have passed on the shots, while poor countries are going begging. Now, a succession of even scarier versions of the coronavirus has emerged. These variants are battering unvaccinated people in hot spots in the United States and around the world, who serve as breeding grounds for a dreaded possibility — a strain that could easily infect and kill vaccinated people.

Vaccine makers are racing to come up with booster shots, but that will only complicate the herculean job of vaccinating enough people to snuff, or at least control, the pandemic.

“Each new variant that ‘succeeds’ does so because it ‘outruns’ all predecessors by being more transmissible,” tweeted Larry Brilliant, an American epidemiologist whose work with the World Health Organization helped to eradicate smallpox. “But the race that matters is not between the variants but between variants and our getting more and more humans vaccinated faster and faster. We need to win this race.”

What is a variant?

As the virus copies itself inside cells, it occasionally makes errors in its DNA code — a typo here, a deleted letter there. Most mistakes are harmful or insignificant to the virus, but a few are beneficial, enabling it to outcompete other strains in circulation.

These “variants of concern” are being spread by international travel. But they are also arising repeatedly, spontaneously, independently — and largely undetected — around the world, sometimes in people with weakened immune systems. Scientists call it “convergent evolution.”

» READ MORE: Do I need a COVID-19 vaccine booster to protect from delta and other variants?

Which variants are being monitored?

Numerous online databases are tracking dozens of variants, but the WHO reports regularly on four that are prevalent in most countries in the world. The variants, nicknamed with letters of the Greek alphabet, are alpha, beta, gamma, and delta. The WHO summarizes studies of important indicators, including the variants’ transmissibility, severity, chance of reinfecting people who recovered from COVID-19, and, crucially, the ability to evade vaccines.

» READ MORE: The Delta variant is in the Philadelphia area, posing a threat to unvaccinated people

Do the vaccines protect against these variants?

So far, the world’s leading vaccines, including the three authorized for use in the United States, have lost little of their protective power even against the four major variants. (However, people who skip the second dose of the Pfizer or Moderna vaccines are still vulnerable, as they’ve reduced their risk of symptomatic infection only by a third.)

And while “breakthrough infections” occasionally occur in fully vaccinated individuals, these are generally mild and not easily spread. As of June 21, the U.S. Centers for Disease Control and Prevention reported only about 4,100 hospitalizations, including 750 deaths, among fully vaccinated people, most of them over age 65. That’s a minuscule fraction of the 607,000 COVID-19 deaths in the United States so far.

Almost half the U.S. population, or 159 million people, has been fully vaccinated.

Studies also suggest that people who have immunity as a result of a past COVID-19 infection are unlikely to get reinfected with the variants.

How dangerous are the variants?

The bad news is that all four variants, and particularly the alpha and delta strains, spread much more easily, primarily in droplets released into the air when an infected person coughs, sneezes, or just talks and breathes.

A BBC graphic, based on research and tweeted by Brilliant, depicts the problem: A person infected with the original coronavirus, first identified in China, infected two to three others on average. In contrast, an alpha infection gets spread to an average of four or five others, while a delta infection spreads to as many as eight other people.

Alpha, first identified in the United Kingdom, also increases the risk of hospitalization, and possibly death, the WHO reported. The virulence of the other variants is still under study.

Where are the variants driving death numbers up again?

In India, where the delta variant is believed to have originated, it has driven a devastating spring surge in cases that has pushed the death toll past 400,000.

In the United States, states with low vaccination rates — notably Missouri, Arkansas, and Mississippi — are being hit hard by the delta variant, although deaths have not spiked. At least, not yet. Missouri hospitals are running out of ventilators and intensive care units are filling up.

How common are the variants in the Philadelphia area?

COVID-19 cases, hospitalization, and deaths have plummeted as vaccination rates have risen. The seven-day average of new cases per 100,000 residents is in the low single digits for Philadelphia, Pennsylvania, and New Jersey.

However, vaccination rates vary, shaped by mistrust, misinformation, and socioeconomics. In Philadelphia, for example, up to 80% of residents of affluent Center City zip codes have had at least one shot, while that is true for only about a third of North Philadelphia residents, according to an Inquirer analysis of city health department data. New cases are concentrated in neighborhoods with low vaccine rates.

The fast-spreading delta variant is not yet dominant among those new cases, but it is growing. In New Jersey, the share of analyzed cases in which the variant was detected rose to nearly 27% in the first week of July, up from 16% the previous week, state health officials said.

In Pennsylvania, the variant was identified in 3.4% of cases sampled over four weeks through June 19, according to CDC data, but its true presence is likely higher.

The delta variant is now dominant in the United States, edging out the alpha variant.

University of Pennsylvania microbiologist Frederic Bushman, whose lab is analyzing COVID-19 patient samples to track the spread of variants, predicted that the transmission dynamics, disparities in vaccination rates, and colder weather will lead to a surge in cases.

“My guess is it will come back in the fall in vaccine ‘deserts,’” Bushman said.