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Before 2020, Vietnam looked particularly vulnerable to a pandemic. The Southeast Asian country, a single-party state with nearly 100 million people, scored low on international assessments of universal health coverage and had relatively few hospital beds for its population, as well as a closed-off political system.

Instead, Vietnam emerged as an early pandemic success story. Long after the coronavirus began to spread in neighboring China, Vietnam maintained low levels of infections and fatalities even as wealthy countries with more robust health systems, including the United States and much of Europe, struggled.

A new study of pandemic preparedness across 177 countries and territories appears to have found a key element in Vietnam's success: trust.

Thomas Bollyky, one of the study's authors, said Vietnam should have failed in the fight against the coronavirus, according to traditional tenets of preparedness.

"What Vietnam does have, that seems to potentially explain what has happened, is that they have very high trust in government - among the highest in the world," said Bollyky, who is a senior fellow for global health at the Council on Foreign Relations, a think tank.

The peer-reviewed study was published Tuesday in the Lancet, a top medical journal, following 10 months of research by Bollyky, his colleague Erin Hulland, a scholar at the University of Washington, and a team of dozens.

The aim of the study was to answer a question that has been dubbed the "epidemiological mystery" of the pandemic: Why did the coronavirus hit some countries so much harder than others?

With countries already looking toward the next major outbreak, and treaties designed to ramp up preparedness for pandemics through new agreements and funding already under discussion, the question is increasingly urgent. But in review data from across the world, the study's authors found that traditional models for pandemic preparedness didn't fit what they were seeing.

"We found no links between covid outcomes and democracy, populism, government effectiveness, universal health care, pandemic preparedness metrics, economic inequality or trust in science," Bollyky said.

These factors were key for pre-coronavirus rankings such as the Global Health Security Index, which in 2019 listed the United States and Britain as most prepared for a catastrophic biological event, like a pandemic — and Vietnam 74th out of 117 countries.

Instead, better outcomes appear to have gone hand in hand with high levels of trust in government and other citizens. Perception of government corruption was correlated with worse outcomes. Researchers measured trust with polling data from the World Values Survey and Gallup.

Rebecca Katz, director of the Center for Global Health Science and Security at Georgetown University Medical Center, and an expert who was not involved in the study, said the research was evidence for what many already argue.

"Trust in government and strength of community engagement is critical to public health response," Katz wrote in an email. "Experts from multiple disciplines have pointed to the importance of risk communication, community engagement and trust as critical to public health messages and policies being implemented. The findings in this paper emphasize just how important this is."

Joshua Sharfstein of the Johns Hopkins Bloomberg School of Public Health said the research showed that "the battle of human being against pathogen was mediated by governments."

"It's really a Chicken Little situation," Sharfstein added. "If people don't believe what the government is saying, then people will be less likely to take the precautions that they need to take."

The study's authors came to their conclusion by looking at standardized infection rates across countries. In reviewing factors outside of government control, such as the age of the population, researchers found clear signs that some governments had done better than others.

The United States, for example, had the second-worst standardized infection rate of any high-income country. A calculation that showed a standardized ratio of infections to fatalities found the United States in the middle of the pack, which could suggest that the country's health-care system is relatively robust even if people were more likely to become infected.

The U.S. government struggled to persuade its population to take measures to combat the spread of the virus, whether it was social distancing or seeking medical intervention like vaccination. Multiple polls have shown that the United States has relatively low levels of trust in government compared with other high-income countries and high levels of political polarization.

"Trust in your government or your trust in others is strongly associated with vaccination rates," Bollyky said. "It is associated with decreased mobility, which is an indicator of social distancing policies."

The Lancet study estimated that if every country had the same level of trust in government as Denmark, one of the countries with more trust among high-income nations, 13% fewer people would have been infected with the coronavirus globally.

"If the same were true for one's trust in others, the reduction would be even larger: 40%, or 440 million fewer infections over the 21-month period assessed in the study," Bollyky said.

The study has its limitations. Comparative international data can only go so far, Bollyky said, and the study focuses only on factors in place before the pandemic.

In addition, later waves of infection have challenged some previous assumptions about how some countries have fared, with some high-trust countries that had evaded outbreaks, including New Zealand, seeing high case numbers. Vietnam, once a relative success story, struggled more with public trust as cases surged in the second half of the year.

Michael Bang Petersen, a professor at Aarhus University, said the study's findings fit into his understanding of how a high-trust country such as Denmark responded to the crisis, but his own research on falling levels of trust in countries amid the crisis left him with a tragic conclusion.

The pandemic has "eroded trust in the government," Bang Petersen said. "It actually seems as if the pandemic has worsened the problem that this study identified."