(McClatchy-Tribune Information Services) -- With swine flu ebbing and immunizations still maddeningly hard to find, one might be tempted to ask: Why bother to even look for vaccine?

Experts say the answer, at least in part, is that the disease almost certainly will come back this winter -- as seasonal flu.

So why are public health officials also still emphasizing immunization with the existing seasonal flu vaccine, also in short supply?

Because, they say, where influenza is concerned, predictions always contain a large element of guesswork.

"No one knows all of what it is able to do," said vaccinologist Kathryn Edwards, a professor of pediatrics at Vanderbilt University School of Medicine in Knoxville, Tenn.

In other words, it is better to be safe than sorry.

While lab-based knowledge of influenza has advanced by leaps and bounds in recent decades -- the new flu's genetic code was determined just weeks after the virus appeared in Mexico last spring -- real-world experience is far more limited.

Scientists' expectations about how this pandemic virus will actually behave in the coming months are based mostly on what happened in the Southern Hemisphere in the last few months, and on past pandemics. There were only three of those in the last century, and they did not all unfold the same way.

But there were similarities.

All of them arrived in waves -- two to four peaks of disease separated by a few months to more than a year.

All of them also changed the mix of seasonal flu strains for years to come, losing their imprimatur only when the next pandemic swept through.

By definition, pandemics occur when a virus to which few people have been exposed appears on the scene.

"It is really sheer dumb evolutionary luck," said Hildegund C.J. Ertl, leader of the immunology program at the Wistar Institute.

With no antibodies from past exposure to either the virus or a vaccine, much of the population has little or no ability to fight the disease.

"The pandemic strain becomes the predominant strain because it can infect more people," said Neil Fishman, head of infectious disease at the Hospital of the University of Pennsylvania.

In succeeding years, as more people get infected or vaccinated, the new strain spreads less efficiently, settling into a seasonal pattern.

What is commonly called seasonal flu actually consists of several strains that circulate through the winter at different times in different communities. Sometimes they appear simultaneously; at other times they do not.

Most seasons include two broad "types" of flu. Type B comes in various strains that tend to appear late in the season and typically cause mild disease.

Type A strains are divided into subtypes that are named according to the versions of two proteins -- abbreviated H and N -- that are involved with reproduction.

Small genetic "drift" from year to year reduces but does not eliminate the immunity developed from past exposure. That is why a new, slightly different seasonal flu vaccine is released annually.

A major genetic "shift" in the virus set the stage for the last three pandemics:

The 1918 flu, a strain within the H1N1 subtype, was devastating. Yet it later settled into a seasonal flu pattern, with small annual changes, until 1957. (It is distantly related to the 2009 H1N1 swine flu, which is probably why older people have more immunity.)

In 1957, a new H2N2 strain caused a pandemic that killed more people than a regular flu but nothing like its predecessor's 50 million to 100 million deaths worldwide.

Slightly changed versions of that strain dominated until 1968, when another new strain -- an H3N2 -- caused an even milder pandemic. Its descendants have circulated ever since, joined in recent decades by an H1N1 that last season developed resistance to Tamiflu.

That era may be ending. Although the regular flu season would not normally start until December and would peak in February or March, scientists say the lack so far of almost any signs of either strain is telling. And in the Southern Hemisphere, which experiences seasonal winter flu during the North's summer, the 2009 H1N1 swine flu dominated.

The pattern is familiar to D.A. Henderson, who oversaw much of the federal response to the 1957 pandemic.

That one first appeared in the United States in early June, then receded over the summer with sporadic outbreaks -- one of the biggest was at a jamboree of 53,000 Boy Scouts in Valley Forge -- that he believes "seeded" the virus around the country.

It reappeared in the fall, for six to eight weeks at a time, in communities nationwide. Mostly the disease was mild. Younger people were more susceptible. The seasonal flu that had previously dominated was gone.

This year's pandemic has been "remarkable," Henderson said in a phone interview, in that "everything that has happened has gone as we forecast."

In 1957, the pandemic wave was over by mid-December. The virus came back in 1958, with a peak in January through March. Henderson and others believe something similar will happen this year.

Henderson, who went on to lead the international campaign to eradicate smallpox and is now a distinguished scholar at the University of Pittsburgh's Center for Biosecurity in Baltimore, raised another potential parallel with this year in a recent paper.

In 1957, drugmakers put a priority on producing pandemic vaccine. It was distributed through a system very different from this year's. And much of it arrived late.

"It was reported that with the end of the fall epidemic, demands for vaccine declined sharply," Henderson and colleagues wrote.

A puzzling increase in deaths from influenza and related pneumonia that winter has never been explained. Whether better vaccination rates would have made a difference is unknown.

Mustafa Yaziciuglu 30, is taking no chances. Unable to get swine flu vaccine from their pediatrician, he and his wife brought their two sons, ages 1 and 2, to the Sayre Health Center in West Philadelphia last Monday.

"I don't want to take a risk for my kids," said Yaziciuglu, a postdoctoral researcher in molecular biology who lives in Drexel Hill.

He is still trying to find vaccine against seasonal flu.

With multiple strains circulating at any given time, seasonal flu vaccine in recent years has contained the three that are predicted to be most common that winter. Because the 2009 H1N1 emerged too late to include, manufacturers interrupted production to make a separate swine flu vaccine -- as it turned out, causing shortages of both.

Public health officials in the Southern Hemisphere have already decided to make the new flu one of their three strains for next season's vaccine. Observers expect a similar decision in the United States.

Although most scientists believe that swine flu will dominate the coming flu season, some of the other strains in the seasonal vaccine will likely still be around.

"It is a very unusual year. It is a year in which some younger children under the age of 10 may need to receive three vaccines," said Richard Scarfone, a pediatrician at Children's Hospital of Philadelphia, referring to the two swine flu doses recommended for a robust response.

"But I think that in this particular year, that is the proper thing to do," he said. "Pandemics don't come along that often."

Vaccine eligibility

Different people are at risk for swine and seasonal flus.

Swine flu: Caregivers and household contacts of infants under 6 months; everyone ages 6 months to 24 years; people ages 25 through 64 with significant medical conditions; pregnant women; health-care and emergency-medical-services workers.

Seasonal flu: Everyone age 50 and older, and ages 6 months to 18 years; caregivers and household contacts of adults age 50 and older, and of children 4 years and younger; everyone living in a nursing home or chronic-care facility; everyone of any age with a significant medical condition; women who expect to be pregnant during the flu season; health-care workers.

Copyright (C) 2009, The Philadelphia Inquirer