We’ve written before about Chile’s strange split personality, in which it’s vaccinating its citizens at a rate higher than almost any other country in the world and yet is still mired in its worst stretch of the pandemic. Scientists have tried to explain that away with different theories. The one favored by Chilean experts is that the country jumped the gun on easing restrictions, believing that the worst of COVID was behind them. After a spike last June, Chile’s cases fell and held stable all the way through November, leading the government to relax lockdowns and allowing people to travel abroad. Chileans ventured out, got infected, then brought it back home. And not just the standard virus, either. Some of the spread is doubtless attributable to variants, like the highly contagious British strain or the dangerous Brazilian one.

Still, that explanation doesn’t sit right. Chileans have pandemic fatigue, sure, but so do Americans. Many U.S. states are out of lockdown right now and haven’t seen the sort of surge Chile is coping with. (Michigan is a notable exception.)

How can that be happening when they’re slightly ahead of America as measured by the percentage of the population that’s received a first dose…

…and only slightly behind us in the current rate at which they’re vaccinating people?

What explains the difference in the state of the two countries? One possibility is that there’s much more natural immunity in the U.S. population than there is in Chile’s population at this stage and that’s helping to hold cases down here. The U.S. ranks eighth among all nations and first among major nations in cases per capita, which is partly due to more extensive testing but probably not entirely. Chile ranks 52nd in the same metric. COVID can’t spread as easily here anymore as it can there because too many of us have already had it. All Chile’s doing now is playing catch up.

But there’s another possibility. On March 2 Jazz wrote a prescient post titled, “China to flood the world with inferior vaccines.” Later that month Ed flagged the surge in Chile as evidence that China’s vaccine, which is ubiquitous there, might be too weak to prevent infections among vaccinated people to the degree that America’s vaccines do. I considered the same theory in a post the following day. Now here comes hard evidence from Chile itself that, yes, China’s vaccine is garbage — at least after a single dose. It does provide meaningful protection after the second dose, but still not to the degree that Pfizer and Moderna do.

On Friday, Chilean authorities released the results of a study of 10.5 million people, showing the vaccine was 16% effective against infection after one dose and 67% effective after a second dose. The study also found it to be 80% effective in preventing death from Covid-19 two weeks after a second dose…

Jaime Mañalich, who was health minister at the start of the pandemic, said the full two-dose regimen must be completed. “The fundamental message is that the start of a vaccination campaign is a very good thing, but you can’t relax the health measures,” he said…

Chile began vaccinating in December, but didn’t ramp up the campaign until the start of February after receiving its first shipment of Sinovac’s CoronaVac vaccines, which account for about 90% of all the shots administered.

Other studies peg the effectiveness of China’s vaccine even lower. A recent study from the University of Chile found it to be 56.5 percent effective after two doses and … just three percent after one. Whatever the true numbers are, it’s so clear that China’s vaccine is inferior to its American competition that the head of China’s CDC recently admitted that their products don’t have very high protection rates and that that problem needs to be solved. The Sinovac shot does help after two doses: Chilean doctors pointed out to the WSJ that hospitalizations among fully vaccinated senior citizens are down and other data shows 85 percent fewer hospital visits among those who’ve had both shots. But Pfizer and Moderna are 80+ percent effective after a single shot and 100 percent effective in preventing hospitalization and death after 28 days by comparison.

What probably happened in Chile was that a bunch of Chileans who’d had their first dose believed they had received a meaningful degree of protection and decided to cut loose a little, only to find out the hard way that they didn’t. Combine those people with the unvaccinated people who brought the virus back from their travels a few months ago and started spreading it around and you have a wave that even a mass vaccination campaign can’t quite hold back. Not until many more Chileans have had both shots, not just one, will the fire start to die down. Compare that to the UK, where the strategy from the start has been to vaccinate as many people as possible with just one dose before worrying about the second. The result:

When the UK decided last December to delay boosters to speed up coverage, it faced denounciations that it wasn’t “following the science”—not just that people could disagree with it, but that it was unscientific. Today, a single UK death while much of Europe faces a deadly surge. https://t.co/eGf2O7jRbv pic.twitter.com/iGzh0CCVvu

— zeynep tufekci (@zeynep) April 19, 2021

The UK also had a strict, months-long lockdown helping to limit transmission, but the reason their one-dose strategy has worked to crush the curve whereas Chile’s vaccination campaign has yet to gather steam may be as simple as which vaccines they’re using. China’s Sinovac product is a bust after one dose. Pfizer, Moderna, and AstraZeneca aren’t.

Exit question: What does Chile do now? Switch to Pfizer and Moderna? Can they even find Pfizer and Moderna doses given the clamoring global demand for them? We could always gift them some of the AstraZeneca doses we’ve been hoarding. In the end, they may follow a simpler strategy, sticking with Sinovac but upping the dosage to produce more antibodies in recipients. Reportedly discussions about administering a third shot to Chileans are happening now.