His statement has since stirred discussion about America's anti-vaccine movement.
A crucial question remains unanswered, however: Is COVID-19 even subject to herd immunity? From universities to sports teams, top experts are still debating this issue.
While the world anxiously awaits a vaccine, the length and durability of the protective immunity it would provide is far more in doubt than one might think.
A new study from China shows that antibodies can disappear in two to three months. The study further found that immunity is shorter for asymptomatic patients than symptomatic ones: The less symptomatic a person is, the weaker the immune response and antibody strength. "Young people who have mild disease or asymptomatic disease, their antibodies may never rise very high," said Sankar Swaminathan, chief of infectious diseases at the University of Utah. "We don't even know if those antibodies are protective."
How does herd immunity work?
Herd immunity occurs when a sufficient proportion of a population is immune to an infectious disease -- either through prior illness or vaccination -- so that contagion from person to person is unlikely.
According to Johns Hopkins, 70%-90% of the population (230-300 million Americans) needs to develop protective antibodies to COVID-19 to achieve herd immunity.
Approximately 2.74 million Americans have tested positive for the coronavirus, over 130,000 of which have died (case fatality of 4.74%). By contrast, the case fatality of the flu in the U.S. is roughly 0.1%.
Absent the existence of a COVID-19 vaccine, any reasonable extrapolation of the data -- even at half the current case fatality rate, means we will see a seven-figure body count that exceeds 5 million deaths before we can attain herd immunity.
How does a COVID-19 vaccine impact herd immunity?
There are three factors that determine if and how well a vaccine can safely bring us along to herd immunity without exposing individuals to the life-threatening consequences of the disease itself.
First is the vaccine's efficacy -- for example, the measles vaccine is 97-98% effective. Dr. Fauci believes that for COVID-19, we are unlikely to get a vaccine that is more than 75% effective.
Second is the vaccine's prevalence of use -- this is where Dr. Fauci's concern about the anti-vaccine movement comes into play.
Fauci noted that "there is a general anti-science, anti-authority, anti-vaccine feeling among some people in this country -- an alarmingly large percentage of people, relatively speaking." He said given the power of the anti-vaccine movement, "we have a lot of work to do" to educate people on the truth about vaccines.
Third is the durability and longevity of the vaccine's induced antibody immuno-response. This is where the two-month to three-month life span of antibodies becomes a concern.
An executive at AstraZeneca, one of the companies working to develop an effective vaccine, told a radio station that he thinks his vaccine might only offer protection for one year.
What do we know about COVID-19 antibodies?
Antibodies are proteins that specifically bind to invading pathogens to neutralize them so they cannot infect the host cell. They trigger a mechanism known as phagocytosis which destroys the virus. IgG antibodies are the most common and can protect us against bacterial and viral infections.
IgG antibody immunity to COVID-19 occurs through contracting SARS-CoV-2, or through a vaccine that produces an immuno-protective response.
Here's where things get dicey. In a scientific brief from April 24, the World Health Organization said, "there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection." They described the notion of an acquired immunity from further infections of the disease for those who had already contracted the coronavirus an unproven and unreliable theory.
"It isn't a uniformly robust antibody response, which may be a reason why, when you look at the history of the common coronaviruses that cause the common cold, the reports in the literature are that the durability of immunity that's protective ranges from three to six months, to almost always less than a year," Fauci said in an interview with JAMA Editor-in-Chief Howard Bauchner.
The United Kingdom, Sweden and Brazil have each allowed herd immunity to inform their approaches to COVID-19 in one way or another -- with severe consequences.
In mid-March, Patrick Vallance, the British government's chief scientific adviser, announced they were taking an approach to COVID-19 that would "build up some kind of herd immunity," but quickly reversed course due to fatal risks.
Similarly, Sweden attempted to attain herd immunity through an approach that ultimately yielded among the highest per capita death rates from COVID-19 in the world, with no measurable associated economic gain, according to the European Commission. Only 6.1% of Sweden's population developed coronavirus antibodies by late May -- a number much lower than predicted.
For reference, a large-scale study out of Spain indicates that just 5% of its population has developed antibodies. This study shows that even though Spain was one of the countries hardest hit by the virus, the presence of antibodies is still only around 5%, which is not high enough to achieve herd immunity.
Most experts say that herd immunity requires at least 60-70% of the population to have antibodies, though this number varies depending on the virus. Because of the toll that this degree of infection would take on the health care system and on the population, the safest way to achieve herd immunity would be through a widely available, widely utilized, effective vaccine.
The poor results from Sweden's controversial approach have led to a formal investigation into the government's actions in response to the public health crisis.
Brazil has fared no better with the second highest number of coronavirus cases in the world.
Surgeon General Dr. Jerome Adams recently advised a leading national physicians' organization that the U.S. is far from reaching the 70%+ infection rate needed to begin having a real discussion about herd immunity. For reference, Adams mentioned his home state of Indiana was currently at an approximate 3% infection rate.
Dr. Adams further reiterated Dr. Fauci's concerns that antibodies appear to be limited in robustness, so it is unknown how significant or effective acquired immunity to COVID-19 may be. While it's still unclear whether or not a person can get infected with COVID-19 more than once, the limited durability of immuno-protective IgG antibodies may suggest an answer the world does not want to hear.
Dr. Jay Bhatt is an internist, Aspen Health innovator fellow and ABC News contributor.
Parag Deven Parikh is an epidemiologist and graduate of the University of Illinois at Chicago School of Public Health.