Lambda variant spreads in South America as scientists seek answers on vaccine efficacy

By early July 2021, a new coronavirus variant was moving through South America with enough speed to catch the attention of the World Health Organization, which had classified it as a variant of interest just weeks before. The Lambda strain, designated C.37, had already been detected in 29 countries, but its presence was most pronounced in the region where it first took hold. In Peru, Lambda accounted for 81 percent of sequenced cases. Argentina reported it in 37 percent of samples collected over a four-month stretch. Chile saw it in nearly a third of recent infections. Brazil had documented three cases and one death. Yet even as the variant spread, scientists were struggling to answer the most basic questions about it: How severe was it? How easily did it transmit? And crucially, would the vaccines already in use around the world still work?

Preliminary laboratory studies from researchers at New York University and the University of Chile offered some reassurance, though the findings remained incomplete. The work, not yet published in peer-reviewed journals, suggested that antibodies produced by Pfizer, Moderna, and CoronaVac vaccines were less potent against Lambda than they were against the original virus strain. But less potent did not mean ineffective. The vaccines still appeared capable of neutralizing the virus. Nathaniel Landau, a microbiologist at NYU's Grossman School of Medicine who studies coronavirus variants, cautioned against overreaction. He noted that much of the concern seemed to stem simply from the fact that a new variant existed and had a new name. There was no evidence, he said, that Lambda would displace Delta, the highly transmissible strain already dominating much of the planet.

The real problem, according to researchers who had tracked Lambda's emergence, was not the variant itself but the gaps in knowledge surrounding it. Pablo Tsukayama, a microbiologist at Cayetano Heredia University in Peru who had documented Lambda's rise, pointed to a fundamental weakness across Latin America: the region lacked robust infrastructure for genetic surveillance of the virus. This meant that as new variants emerged, scientists could not quickly sequence enough samples to understand their true prevalence or characteristics. The result was a vacuum of information that bred speculation and concern. Ricardo Soto-Rifo, a virologist at the University of Chile who had also studied Lambda, echoed the point. Compared to other variants, he said, there was simply not much known.

What the preliminary studies did reveal about Lambda's biology was worth noting. Like other variants, it bound more tightly to human cells than the original strain, a characteristic that could make it more transmissible. The reduced potency of vaccine-induced antibodies was real, though Soto-Rifo emphasized that this alone did not mean vaccines had lost their protective power. The human immune system, after all, had multiple layers of defense. Even if antibodies were less effective, T cells and other immune components could still provide protection. Human trials specifically testing vaccine performance against Lambda had not yet been conducted, and Soto-Rifo said such studies would be necessary to draw firm conclusions.

Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle, offered perspective on Lambda's global threat. The variant had been circulating for some time, he noted, yet it had barely reached the United States—unlike the Gamma variant, which had first emerged in Brazil and spread more widely. He did not view Lambda as a concern comparable to Delta, and he did not expect it to become globally dominant. The focus, he suggested, should remain on Delta, the strain already reshaping the pandemic's trajectory worldwide. For now, Lambda remained a regional phenomenon, one that illustrated both the speed at which new variants could spread and the limitations of the surveillance systems meant to track them.