Amidst all the frenzy over the bird flu virus that's raging among poultry and has killed 71 people since its onset, the issue of human immunity tops the list of grave concerns.
Because the avian flu that’s spreading rapidly among birds in Asia and now parts of Europe is a never-before-seen strain of H5N1, human beings have no natural immunity to it — and thus no way for their bodies to fight it off should it mutate into a disease that can be passed from person to person.
“People do not have any defenses against this virus because it is entirely new,” said Jennifer Morcone, a spokeswoman for the Centers for Disease Control (CDC). “It’s not like the other flu viruses that have circulated in the past.”
Scientists and physicians know for sure that human beings don't have any natural immunity to H5N1 because they've measured and analyzed the antibodies in patients' blood serum.
To date, everyone who has fallen ill with this form of avian flu has caught it by handling or coming into close contact with infected birds. So far, there have been 139 confirmed human cases reported since 2003, when the illness was first discovered, and 71 deaths — a 51 percent mortality rate, according to the World Health Organization’s Dec. 16 figures.
In order for bird flu to mutate so that people can catch it from each other, an infected person has to be carrying another type of human-to-human flu bug at the same time that he or she catches the bird flu, so that the two influenza strains can swap genes.
"The avian flu picks up some genes from the other flu," said Dr. Herbert L. DuPont, director of the Center for Infectious Diseases at the University of Texas School of Public Health. "This has happened three times in the past 100 years."
All three pandemic flus of the 20th century originated from birds. The legendary 1918 Spanish flu took between 20 and 50 million lives, up to 675,000 of them in the United States; the Asian flu of 1957 killed 2 million worldwide, 70,000 in the U.S.; and the Hong Kong flu of 1968 killed 1 million worldwide, 34,000 in the U.S.
With that history in mind, what worries immunologists and other health experts is the fact that the existing form of avian flu is voraciously infecting birds across a wide swath of the globe.
The H5N1 virus initially emerged in poultry in 1996 and the first human cases and deaths were reported in 1997 in Hong Kong. Alarming, widespread poultry outbreaks began happening in 2003, which was also the onset of more human cases in Hong Kong and Vietnam.
Since then, the disease has spread to Thailand, China, Indonesia and Cambodia — where the other human deaths have occurred — and also to Japan, Laos, Korea, Malaysia, Croatia, Kazakhstan, Mongolia, the Philippines, Romania, Russia, Turkey and the Ukraine.
"It is a real threat because we know this is a virus that just this year has spread to three different countries in Asia," Morcone said, referring to China, Indonesia and Cambodia, the countries where human deaths have been reported in 2005.
That means it's just a matter of time before this H5N1 will travel closer to home.
“Eventually, when you have migrations of birds, you’re going to get some cases in North America,” said Dr. Gilbert Ross, medical director at the American Council on Science and Health.
In addition, unlike other flu viruses that are generally only fatal among the elderly and infirm, this influenza strain has been killing younger people — much like the 1918 outbreak. That is in part because most of those afflicted were bird handlers in their 20s to 40s, but it also could be an indication that this strain is similar to the devastating disease of the early 20th century, which was an H1 flu.
"Fifty percent of the people who died in 1918 were between the ages of 20 and 40," said Dr. Ed Septimus, medical director of infectious disease at the Memorial Hermann Healthcare System, a cluster of hospitals in the Houston region. "We're seeing a similar mortality rate and younger people being infected and dying."
Influenza is notorious among diseases for its frequent changes, or drifts, in strain, meaning it is difficult to immunize against via one of the two main ways immunity occurs — by vaccine or by infection with the illness.
The run-of-the-mill flu that makes an appearance every year is constantly mutating, so a new vaccine needs to be developed annually to combat whatever strain is the most prevalent in the community during that season. It's a challenge to doctors and scientists, who sometimes come up with a less effective vaccine because of the unstable nature of influenza.
“Your best protection is when the vaccine matches the strain that is circulating in the community,” Septimus said. “In the past few years, the predominant flu bug in the community was not in the vaccine; it was a mismatch. We guessed wrong. But that still gave about 50 percent protection. If it had been a match, it would have been about 80 percent.”
The drifts in the regular cold-weather flu differ from the larger "shifts" in the virus, which happen when the hemoglobin — the H in the scientific name for the strain — changes entirely. This current H5N1 avian flu is new because an H5 had never been seen before.
"When the hemoglobin changes, it's a big deal," DuPont said. "When it shifts and goes to a completely different hemoglobin, there's no existing immunity and we have pandemic flu."
The so-called "shift-and-drift" nature of the flu also means that human beings who get their shots or catch that particular bug are only protected from that one specific strain during that one specific time. Unlike immunity to most other diseases, immunity to influenza is fleeting.
“The protective immunity against flu is four months from the influenza vaccine — you have protective levels of the antibody for approximately four months,” DuPont said. “That’s why people who get the vaccine in the early part of the season can still get the flu in the later part of the season.”
Immunity is built up when parts of the virus in question are introduced into the body, either because the person has become infected or has been vaccinated. The human system then builds up antibodies to ward the illness off.
There are two types of human vaccines: live vaccines and killed ones. Most flu vaccines are killed, meaning the agent in them is dead and can no longer be grown in a lab, but still produces antibodies and protects against the virus. Vaccines for illnesses like measles, mumps and rubella are live, made with a less virulent strain of the virus that's processed in such a way so as not to cause sickness, while still providing immunity and having the ability to keep growing in a lab.
Many people are exposed to sicknesses of various stripes without ever having any symptoms, as is the case with “the kissing disease” known as mononucleosis. Of every 1,000 people infected with mono, only five unlucky ones actually fall ill from it — but all are immune, according to DuPont.
“The most common way to build up immunity is to have an infection with the agent, and most agents do not produce clinical disease when they infect,” he said.
Immunity to illnesses like mono, chicken pox (which is the varicella virus), measles, mumps, polio and smallpox is generally long-lasting among people, with those infected or vaccinated having protection that can last at least a generation, if not a lifetime.
In fact, even when a human immune system hasn't seen a virus for decades and then is reintroduced to it decades later, "memory cells" in the body will often recall it and begin to quickly develop antibodies to fight it off, meaning the person will either not get sick at all or will have a much milder form of the disease.
"Sometimes, seeing a virus that's similar will wake up a dormant immune system and produce antibodies very quickly in response to a challenge from a virus," Septimus said. "It may have an effect on how severely you become ill and the expression of the illness."
Human immunity does diminish slowly over time, but with most diseases other than influenza, exposure to an illness provides at least partial immunity for years even when it has decreased from what it was originally. Flu, of course, is the black sheep.
But experts are trying to soothe those frightened by the current avian influenza that has an increasingly large part of the world in its grips, saying there is hope for a flu shot to be developed that will successfully ward it off.
"We will see an effective vaccine, just like we've seen effective vaccines for nearly every other communicable disease except HIV and malaria," said the American Council's Ross.
He and others in the field urge that in the meantime, the immediate focus should be on taking precautions, both against bird flu and the regular garden variety that's crept up on us again this wintry season.
"If they're traveling to Asia, we recommend that people don't visit live bird markets and don't go to bird farms, be mindful of the food they eat and mindful that food is cooked," said the CDC's Morcone.
Avian influenza aside, the best advice could also be the best medicine: Wash your hands frequently, especially when you're sick with the flu or a flu-type bug, stay home until you get well and cover your mouth when you cough and splutter.
"Let's not get hung up on bird flu, which may not happen — and if it happens we don't know when," Septimus said. "Do what you're supposed to do this year, and let the scientists and government work out a plan for the potential of a pandemic."