On one side of the island of Hispaniola lies Haiti, among the Western Hemisphere's last nations to offer its population a first dose of COVID-19 vaccine. On the opposite side, the Dominican Republic is one of the first countries to offer a third.

The nation of almost 11 million people is rushing to provide boosters as the pandemic rages in its destitute neighbor, Haiti, defying criticism from global health organizations that no scientific evidence shows they're necessary.

The Dominican Republic has had one of Latin America's strongest immunization efforts, using mostly Chinese Sinovac inoculations to protect almost 50% of its population with at least one dose. But residents are starting to question Sinovac's efficacy compared with mRNA vaccines. Since the government said June 30 that it would provide mRNA booster shots, almost 289,000 people have been inoculated a third time, including President Luis Abinader.

Meanwhile, Haiti has had one of the region's weakest vaccination campaigns. Even as it wrestles with the recent assassination of President Jovenel Moïse, the Western Hemisphere's poorest country received its first shipment of COVID-19 vaccines only July 14.

Across the globe, relatively wealthy countries are struggling to use all their vaccines, while poor nations go almost entirely without. Low-income countries account for almost 20% of the population yet have received only 2% of vaccines, according to the Bloomberg Vaccine Tracker.

Arthur L. Caplan, director of the NYU Langone health system's medical ethics division, said wealthier countries must help fight a virus that recognizes no borders.

"There have been a few examples of sharing vaccines with a neighbor, but not many," Caplan said. "We have failed, miserably, to distribute the supplies we have to those most in need. That's a total moral failure."

The Caribbean island of Hispaniola, which the Dominican Republic and Haiti share, is a microcosm of global disparity.

At the outset of the pandemic, Haiti closed its borders and kept its citizens relatively safe, while the tourism-dependent Dominican Republic -- nearly 10 times richer -- became a viral hot zone. A year ago, it had reported more cases than any other Caribbean nation. Now, the countries have traded places.

The nations have a historically complex relationship as they attempt to coexist on the West-Virginia sized island. Many Haitians travel east to seek work and better living conditions, and illegal immigration is one of the main contributors to the tension. Caplan believes Haiti could engender new variants that could bleed over the Dominican border.

The Dominican Republic is joining Bahrain, Indonesia and Thailand in augmenting Sinovac and Sinopharm campaigns with mRNA shots -- even though experts agree that the Chinese shots lower hospitalizations and deaths effectively.

"What we are looking for is that extra coverage against the variants, to increase the antibodies among the patients and to avoid reinfection and hospitalization," Minister of Public Health Daniel Rivera said at a press conference last month.

The campaign began Feb. 16. The government has established more than 1,300 vaccination sites across its 31 provinces, 19 near the border.

The third doses, mainly from Pfizer and AstraZeneca, are officially prioritized for health-care workers and people with underlying conditions, though they are widely available to all. Those who want one must wait at least a month after their second, and receive a different vaccine from the previous two. Everyone with a permanent Dominican address is eligible.

The nation is now recording about 400 new daily cases, down from the 1,300 peak at the beginning of June. The improvement is crucial to reviving its tourism-dependent economy. The republic is finally seeing more visitors, with about 20% more last month compared with May. The International Monetary Fund expects the economy to grow 5.5% this year.

"Economic prospects are encouraging, underpinned by the reopening of the tourism industry and the sustained pace of the vaccine rollout," strategists led by Regis Chatellier wrote in a note Tuesday encouraging clients to buy Dominican bonds. "Despite rising debt and a higher fiscal deficit in 2020, the economy massively benefited from the cushion this provided."

But global health organizations are less impressed. The Pan American Health Organization and World Health Organization urged the Dominican Republic to delay a third dose.

"It is vital that we shift the conversation away from booster shots," said Carissa F. Etienne, director of the Pan American Health Organization. Health officials across the world must guarantee that "everyone, everywhere and particularly health-care workers and those most vulnerable to severe disease are protected first."

The office of Vice President Raquel Pena, who is overseeing the vaccine program, didn't answer questions about the third-dose initiative.

Haiti didn't start a vaccination campaign for its 11 million people until after the July 7 assassination of its president, which is still under investigation amid violence and political instability.

Official numbers show Haiti has reported almost 20,000 COVID-19 cases and more than 500 deaths, but doctors agree these numbers are far understated. Haiti has seen high hospitalization and death rates, and crowded shelters could become hot spots for COVID-19 transmission.

"Limited supplies and violence are also hindering the ability of health workers to safely care for patients in need. In some cases, patients may be avoiding seeking care due to safety concerns," Etienne said in a press conference July 14.

Ralph Ternier, a doctor who is director of community care and support for Partners in Health Haiti, which treats the poor, is worried. "The country might not have the resources to respond, that's why right now we are really focusing on the vaccine," he said.

Pedro Urena, a Dominican cardiologist, sees danger in the highly permeable Haitian-Dominican border. Urena has been an outspoken critic on the third-dose initiative, and believes Dominican resources could have been used differently.

"Haiti has a very bad COVID situation," he said. "They have a very high hospitalization rate and a very high mortality rate. The problem is that since they don't have very trustworthy statistics, sadly we can't really know the magnitude of the problem. But I understand that makes it so we have to be even more careful with the migration flow into this country."

Caplan said helping your neighbor while protecting yourself isn't mutually exclusive. In fact, it's essential.

"You can just sort of figure out international policy that would get the vaccine everywhere, but we just seem unwilling and unable as a world to do it," he said.