Experts: New H1N1 Swine Flu Pandemic Unlikely

National Institutes of Health Scientists Say Many in U.S. Are Already Immune

Medically Reviewed by Laura J. Martin, MD on September 30, 2010

Sept. 30, 2010 – Some two-thirds of Americans may already be immune to H1N1 swine flu, making an explosive new wave unlikely.

High vaccination rates this flu season, especially among children and young adults, might even drive the pandemic bug to extinction, speculate top researchers at the National Institutes of Health.

"Clearly, a large percentage of the U.S. population must already be immune to pandemic H1N1, reducing opportunities for explosive pandemic spread in the future," write David M. Morens, MD; Jeffery K. Taubenberger, MD, PhD; and Anthony S. Fauci, MD.

"History suggests that pandemic H1N1 likely faces extinction unless it mutates," they add.

Fauci is director of the National Institutes of Health. Morens is his senior advisor, and Taubenberger is a flu expert and senior investigator at the National Institute of Allergy and Infectious Diseases.

However, the H1N1 swine flu bug isn't yet gone, and the most likely scenario is that it will continue to haunt us for a few years, even if it can't reignite the pandemic.

"It is noteworthy that other post-pandemic [flu] viruses have continued to cause various rates of excess mortality among younger persons for years after pandemic appearance," Morens, Taubenberger, and Fauci warn.

H1N1 Swine Flu Immunity High in U.S.

The National Institutes of Health researchers calculate that more people may be immune to H1N1 swine flu than previously appreciated:

  • Even before H1N1 swine flu appeared, some 19% of the population had pre-existing immunity, likely due to some combination of exposure to related viruses and related vaccines.
  • About 20% of the U.S. population received the H1N1 swine flu vaccine.
  • About 20% of the U.S. population got swine flu.
  • It's possible that the 1976 swine flu vaccine offered some protection against H1N1 swine flu, even though it did not raise protective antibodies. If so, 8.3% more of the population is protected.
  • People without the usual kind of antibodies that protect against flu may have other kinds of protective antibodies or other kinds of immune protection against the pandemic flu bug.

This means that at least 59% of Americans can't get H1N1 swine flu unless it mutates. Over 67% of the population may have some kind of protection.

The 2010-2011 seasonal flu vaccine protects against H1N1 swine flu. If vaccination rates are high in the most vulnerable population -- children and young adults -- we may be able to say good-bye to the pandemic virus once and for all, the researchers say.

All bets are off if the H1N1 swine flu bug mutates, which it can do by a number of different mechanisms. But it hasn't done this so far, suggesting that it may have limited ability to change.

Indeed, previous H1N1 viruses have not mutated very rapidly. On the other hand, they've managed to hang on and even come back despite very high levels of immunity all around the globe.

And it's frustratingly unclear why, despite their limited ability to mutate, H1N1 viruses have hung on, even though they face competition from the seasonal H3N2 virus, which has been mutating very quickly since it appeared in 1968.

The Morens report appears in the online journal mBio.