To many, the idea that those living in areas hit by the novel H1N1 flu pandemic this summer may have greater protection against the disease in the fall may be a comfort.
But some experts in infectious disease and public health fear a report in the New York Times today suggesting such a theory could cause the public to become complacent about the illness.
At the heart of the debate is the concept of "herd immunity" -- the idea that in a population in which a sufficient percentage of individuals are immune to a given disease, others within the population who have no immunity are still protected from infection by those around them.
It very well could be a concept that applies to the new H1N1 flu, noted Dr. William Schaffner, chairman of preventative medicine at Vanderbilt University in Nashville, Tenn.
"The story is based on reasonable surmise," Schaffner said. "The 'reluctance' of public health officials is because they are careful, data-based persons who are loathe to speculate even if the speculation has reasonable foundation."
But could such speculation lead the public to take the pandemic illness less seriously than they should?
"The idea that prior infection, whether symptomatic or asymptomatic, would elicit immunity ... is not surprising or unusual," said Dr. Edward Janoff, professor in the Infectious Diseases Division at the University of Colorado Health Sciences Center at Denver.
But, he added, "Since only a minority of the population has been exposed and infected so far, even if a substantial minority, there is no reason to withhold influenza vaccine or to avoid getting it. We don't really know who those exposed or immune persons are."
Dr. Carlos del Rio, professor of medicine in the Division of Infectious Diseases at the Emory University School of Medicine in Atlanta, agreed that while the theory bears consideration, it may be hard to determine whether it is reflected in reality.
"This is a very interesting observation, but I would not say that those cities that had the first initial wave are yet off the hook," he said. "The truth is that we really don't know."
Specifically, del Rio pointed out, an estimated 500,000 people in Mexico City became infected between April and June. Estimates from New York City hover around 1 million people infected.
"Is that enough to provide herd immunity?" he asked. "Maybe."
Public Health Officials Sound Off
But for many public health experts, "maybe" is not good enough for public consumption. Public health officials from around the country registered their concerns over the mention of the idea that herd immunity may provide a reason to worry less about the pandemic influenza strain.
"These theories are of scientific interest but don't change the basic fact: H1N1 is a serious disease with an unusual pattern, said Jonathon Fielding, Los Angeles County director of public health and health officer. "We have already had over 50 deaths from H1N1 in Los Angeles County since the spring, and all the indicators show a continuing increase in flu infection rates. We must not dilute the clear message that, at least for the priority groups, vaccine is strongly recommended and that the vaccines are safe and effective."
"My concern is that this is speculation and will only confuse the public," said Dr. Judy Monroe, state health commissioner of Indiana. "There are other respiratory viruses that circulate all the time and unless individuals had confirmatory testing they do not know if they had the 2009 H1N1 influenza."
Dr. Karen Smith, public health officer with the Napa County Health and Human Services in Napa, Calif., said that the idea that the preliminary wave of the new flu strain could have contributed to herd immunity is an interesting one. But she said that it must be viewed cautiously.
"In general, I am very much in favor of the press discussing these sorts of issues and 'theories' as long as that discussion is complete and provides all sides of the issue," Smith said. "Unfortunately, it seems to me that the pressure to provide three-minute-or-less news stories designed to capture the interest of the audience does not lend itself to providing the kind of balanced information the public both needs and deserves."
Other public health officials shared similar concerns.
"It is definitely appropriate to point out that, despite these data, an overwhelming majority of people were not infected by this virus," said Dr. Tony Marfin, Washington state communicable disease epidemiologist. "The conditions for widespread transmission may simply not be in place in Seattle [and] Washington yet."
False Sense of Security Could Pose Risk
"I'm concerned that this could give people a false sense of security, leading some to eschew vaccination, with the consequence that they contract the flu and spread it to others," said Dr. Paul Cieslak, communicable disease program manager for the Public Health Division of the Oregon Department of Human Services. "I acknowledge the theoretical possibility that some areas of the country have had enough influenza to develop herd immunity, but the bottom line is that we don't know which areas they are."
Dr. Timothy Jones, state epidemiologist with the Tennessee State Health Department, noted, "Unfortunately, it's always dangerous to have scientific debates in the public domain, which too often seems to lead to the population -- or media -- getting the wrong message. ... It is very worrisome to me that this is all going to give the impression that 'If the flu has gone through, then I'm probably immune and don't need the vaccine.'"
And Dr. Kelly Moore, medical director of the Tennessee Department of Health's immunization program, said, "My impression is that some people are looking for any excuse to decide not to be vaccinated, and such people would seize on this as justification."
Herd Immunity Not a Simple Issue
Complicating the issue is that herd immunity is not always a cut-and-dried issue. Specifically, said Michael McDonald, president of the not-for-profit global health care organization Global Health Initiatives, Inc.
"Given that the most susceptible populations and the most likely transmitters of diseases in some areas with high population density were hit harder in the first wave, it is likely that they would have less rapid transmission in the second wave, especially within specific social networks, [for example], students within a specific high school, or soldiers within a specific barracks, family members, or employees within a specific building," McDonald said.
"However, that may also mean a delayed or diminished second wave, rather than no second wave in those geographic areas, because the infection may travel through the same geographic area by going through different social networks," he added. "Susceptibility in different social networks may change with weather changes."
Most Still Need Vaccine, Experts Say
"With influenza, one must prepare for the worst and hope for the best," said Dr. Pascal Imperato, dean and distinguished service professor of infectious disease at SUNY Downstate Medical Center in New York. "That said, there is a real risk that many will opt not to be immunized if they perceive that an epidemic is unlikely. Such decisions will be reinforced by widespread belief that the vaccine carries risks."
"It's an interesting hypothesis but difficult to prove," said Dr. Howard Markel, infectious disease expert and medical historian at the University of Michigan at Ann Arbor. "Don't forget that perhaps 5 percent of New Yorkers, at most, contracted H1N1 last spring. That means that at least 95 per cent of those living there are still susceptible.
"The bottom line is that most of us still need vaccine. The tragedy of such media speculation would be for Americans to be under a false impression that they were immune when they are not."
And the consequences, some say, are too heavy to risk.
"The type of speculation that public health authorities are engaged in that raises the possibility that areas such as New York City may have experienced so much flu during the first wave in the spring and summer that they will have very mild second waves or no second waves all is very appropriate after flu season is over, or in private discussions in conference hallways," said Bill Muraskin, professor of urban studies at Queens College in New York. "But to raise them in a way that could easily become public now seems ill advised. ... These types of speculations are not helpful."
The ABC News Medical Unit contributed to this report.