The race is on between COVID-19 vaccinations and the continuing evolution of coronavirus variants that threaten to undermine them.
As vaccination ramps up in the United States and cases decline, people are letting their guard down, including those who are not vaccinated.
But public health experts are urging people not to let loose just yet.
The virus is not done evolving, they note. Some variants have already emerged with traits that weaken the protection the vaccines provide against the virus. The more it spreads, the more chances it has to get better at ducking the vaccines' defenses.
Power of vaccination
So far, the vaccines are proving their worth.
Until recently, the only results available were from tightly controlled clinical trials. Now that the vaccines are rolling out, real-world studies are rolling in.
"You're always worried, if you start giving these doses, if they're not handled right, et cetera, et cetera, will they function as well? And yes, they have," said University of Michigan School of Public Health epidemiologist Arnold Monto, who chairs the U.S. Food and Drug Administration's advisory committee for COVID-19 vaccines.
The Pfizer-BioNTech and Moderna vaccines were 90% effective in preventing any kind of infection, with or without symptoms, in a U.S. Centers for Disease Control and Prevention (CDC) study of health care and other front-line workers. They even provided 80% effectiveness after just one dose.
Another CDC study of nursing home residents, who are among the most vulnerable to serious illness and death from COVID-19, found that just the first dose of the Pfizer-BioNTech vaccine gave 63% protection.
The elderly have been hit hardest by COVID-19. More than 80% of all COVID-19 deaths in the United States have been among people over age 65, according to the CDC.
Now that more than half of all senior citizens have received at least one dose of vaccine, deaths and hospitalizations are down sharply nationwide.
"It's good news with regard to the power of vaccination," CDC Director Rochelle Walensky told reporters on Monday.
But the number of people under 50 hospitalized with COVID-19 has ticked up in recent weeks as cases have increased, she noted.
Pandemic fatigue, improving weather and loosening government restrictions have led to an increase in infections and a sense of deja vu among experts.
"There is a lot of concern that we're not doing the things that we should be in order to keep this virus in check," said epidemiologist Jeffrey Shaman at the Columbia University Mailman School of Public Health.
Walensky told reporters last week that she felt a sense of "impending doom" over the direction the trends were headed.
Some experts are concerned that a fourth wave of infections is starting, driven by more infectious variants of the virus.
While most models do not show a spike of the magnitude of previous ones the United States experienced, "there are signs that the decline [in cases] is slowing," said Michael Li, part of the COVID Analytics group at the MIT Operations Research Center. "So, we're sort of ending back into a plateau stage again."
What especially concerns scientists, however, is that the longer the virus circulates, the more chances it has to mutate into a more dangerous form.
The variant that first appeared in South Africa is perhaps the most concerning so far. It contains an array of mutations that allows the virus to evade the immune system better than the original strain. In a clinical trial, Johnson & Johnson's vaccine was 57% effective in South Africa, compared to 72% in the United States, where this variant was not common. On the plus side, it was 85% effective against the most severe cases in all locations.
AstraZeneca's vaccine fared even worse against the South African variant. It was only 10% effective against mild-to-moderate cases, though severe cases were not studied.
Another strain, first spotted in Brazil, spreads faster and also seems to be able to infect some people who had already been infected before.
Another, recently reported from Tanzania, contains the most mutations recorded so far, including many of the same ones as the South African variant.
Not all worrisome variants are found overseas. Two strains found in California are on the CDC's list of variants of concern, and two identified in New York are also of interest. They have some of the same mutations as the South African and Brazilian variants. Antibody treatments do not work against them.
"We're in a bit of an arms race," Columbia's Shaman said. "We're going to have to make new treatments, new monoclonal antibodies, new variants of the vaccine, potentially, if we see more and more of these variants arising."
And the more the virus spreads, he said, the more variants will arise.