In November 2019, before the world had even heard of the novel coronavirus, 14 children on the Pacific island nation of Samoa were hooked up to ventilators. They were each fighting for their lives against a different, but also highly infectious viral disease: the measles.

During that outbreak, 81 Samoans died, and they were all deaths that could have been prevented with a safe and effective vaccine.

It's this anecdote, among so many others of people who've died because they opted not to get their recommended shots, that has the World Health Organisation worried that we may never get rid of COVID-19, even if and when there is a vaccine.

"I don't think anyone can predict when or if this disease will disappear," the WHO's Executive Director of Health Emergencies Mike Ryan said during a press conference on Wednesday.

"We do have one great hope, if we do find a highly effective vaccine that we can distribute to everyone who needs it in the world. We may have a shot at eliminating this virus.

But that vaccine will have to be available. It will have to be highly effective. It will have to be made available to everyone, and we will have to use it."

Ryan's curmudgeonly assessment came just hours after the WHO's chief scientist Soumya Swaminathan told the Financial Times that it may take "4 to 5" years to "control" the coronavirus, adding there is "no crystal ball" to know if things will get better or worse in this outbreak, or whether we'll be able to develop an effective vaccine at all.

Even getting a vaccine on the market, Ryan agreed, is still a "massive moonshot."

"This virus may never go away," he said.

Without a vaccine, it could take 4 to 5 years to control the COVID-19 outbreak

Most people in the world have not yet been exposed to COVID-19, which means the world is still in a very vulnerable spot.

"The current number of people in our population who've been infected is actually relatively low," Ryan said, alluding to recent blood tests being taken around the world to look for antibodies, which so far (though the tests are still somewhat unreliable) suggest less than 10% of the world has been exposed to the coronavirus.

In the absence of a vaccine, then, it could take many years for the disease to settle "into an endemic phase," Ryan said, where many people have been exposed, and it's circulating like other seasonal viruses do.

In the US, Dr. Anthony Fauci, who directs the National Institute of Allergy and Infectious Diseases, where scientists are at work developing a COVID-19 vaccine, told members of the US Senate on Tuesday that a vaccine will certainly not be ready by the time university students head back to class this fall.

"Even at the top speed we're going, we don't see a vaccine playing in the ability of individuals to get back to school this term," Fauci said.

If a vaccine does become available in the years ahead, there are also plenty of political, financial, logistical, and human-scale issues to solve about how it will be distributed fairly and cheaply, and whether there will even be enough glass vials and needles to go around.

"Science can come up with a vaccine," Ryan said, "but if someone's going to make it, then we've got to make enough of it that everyone can get a dose of it, and we've got to be able to deliver that, and people have got to want to take that vaccine. Every single one of those steps is fraught with challenges."

Only about half (53%) of US adults aged 35-44 said definitively that "Yes, I would get vaccinated" against COVID-19, if a vaccine were to become available, according to a Morning Consult poll taken earlier this month.

Likewise, the percentage of US adults who feel "very comfortable" with vaccinations is declining, and the share of people who say they're "not at all comfortable" with vaccines is on the rise, even since January, in the midst of this devastating pandemic, according to a CivicScience survey.

"Forgive me if I'm cynical, but we have some perfectly effective vaccines on this planet that we have not used effectively for diseases we could eliminate and eradicate, and we haven't done it," Ryan said.

"We've lacked the will, we've lacked the determination to invest in health systems to deliver that, we've lacked the capacity to sustain primary health care, at the front end."

This article was originally published by Business Insider.