The race for a coronavirus vaccine to help end the pandemic has created an escalating demand for an essential resource: monkeys.

Before drug companies call on human volunteers, monkeys are used in preclinical trials to test a vaccine’s safety and effectiveness. But with more than 100 vaccines in development around the world, there aren’t enough monkeys to go around.

“There is a shortage,” said Dr. Skip Bohm, associate director and chief veterinary medical officer of the Tulane National Primate Research Center.

Like other aspects of society, the pandemic has underscored an already existing problem. Nonhuman primate research centers have been strained in recent years because of restrictions on imported monkeys from countries like China and India, and a lack of funding to support domestic breeding.

“We’ve always been in a state where we were always very close to the level of production to meeting the demand for research, and that has been the status for several years,” Bohm said. “When the COVID pandemic came about, that just pressed us even further.”

While animal rights advocates condemn using primates in experiments, researchers say it’s essential for development of medical science.

The National Primate Research Centers try to use the smallest number of animals necessary for a valid scientific result, according to the centers’ website. Unfortunately, it’s not currently possible to completely replace animal models with computer simulations or cell cultures.

“We all hope there’s a day we don’t have to use animals in research but right now … not all humans are going to submit for an examination where they get regular x-rays, regular CT analysis or blood analysis,” Bohm said.

Rhesus monkeys, or Rhesus Macaques, are the most commonly used monkeys for preclinical trials because they share about 93% of their genes with humans, according to the National Primate Research Centers.

Jay Rappaport, director and chief academic officer of Tulane’s National Primate Research Center, says the species has been used in research for years, so scientists are most familiar with how its immune system combats pathogens, especially coronaviruses.

“Their immune systems and immune responses are very similar to what you see in humans and they can give you a very good idea of safety and efficacy in vaccines,” he said.

The Washington National Primate Research Center breeds pig-tailed macaques, which could play an important role in second generation vaccines, according to Deborah Fuller, the center’s division chief of infectious disease and translational medicine. This species of monkey more resembles humans when it comes to pregnancy and underlying health conditions.

According to a 2018 analysis by the National Institutes of Health, the national primate centers’ projected demand for monkeys would increase by 20% to 50%. Most centers were not equipped to accommodate that kind of increase – then the pandemic hit.

Tulane’s primate research center has about 5,000 monkeys but only about 500 are used for research in a normal year because of age, health and colony dynamics. This year, Bohm estimates the same number of primates might be needed across the centers just for COVID-19 research alone.

To satisfy the demand, NIH and research centers have had to collaborate more closely than ever. NIH created a committee to prioritize COVID-19 research while centers developed master protocols to optimize research, including sharing control groups.

“By doing this, this saves us probably half of the animals,” Rappaport said.

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While working together alleviates some pressure on primate centers, it doesn’t help with another limited resource strained by the coronavirus pandemic: space.

Scientists use animals for preclinical trials because they can introduce them to the virus, which is considered unethical for human trials. In order to do that safely, the animals are kept in special facilities called Animal Biosafety Level 3 labs.

While many facilities have such labs for smaller animals like mice and hamsters, very few are large enough to accommodate monkeys. Once scientists finish developing a vaccine, they’re forced to wait until a lab is available.

“The real bottleneck is the access to the ABSL 3,” Fuller said. “(Scientists) are ready and their products are ready, but now they’re twirling their thumbs.”

Rappaport said building the Level 3 labs can cost anywhere from $75 million to $100 million. Some primate centers are upgrading their facilities to “bio bubbles,” using existing structures to comply with requirements without having to build a lab from scratch.

“It’s a way that they’ve been able to use existing space and convert it quickly,” said Sally Thompson-Iritani, the Director of the Washington National Primate Research Center. “Currently the bio bubbles are in such high demand because there are other facilities looking for that.”

Thompson-Iritani inquired about a bio bubble for her research center in Washington state and was told it would be six to nine months before she could get in touch with someone to evaluate her facility.

While experts say there’s little that can be done to fix the shortage of monkeys and lab space for COVID-19 research, there’s still time to prepare for the next pandemic. Primate centers and other institutions require more funding to expand breeding colonies and build labs, they say.

Bohm said some research centers have discussed the possibility of an animal reserve available when “some unforeseen emergency comes up.”

“One thing is for sure this is not the last pandemic that we’re going to see,” he said. “That’s inevitable.”

This article is originally published at USA today.