Interestingly, the H1N1 virus suddenly reappeared in the 1970s. Since then, seasonal influenza has been produced by a combination of the H3N2 virus and the H1N1 virus. Thus, annual influenza vaccines target both the seasonal H1N1 virus and the virus derived from the 1969 pandemic, along with the influenza B virus.
Epidemiological data going back over a hundred years show that influenza pandemics occur about every 30 years. Although the reasons for this recurring time interval are not understood, the data are strong enough that, especially since the late 1990s, experts have anticipated the development of the next pandemic.
The H5N1 avian influenza that emerged in Hong Kong in 1997 fortunately has not mutated enough to be easily transmissible among humans. Experts have been concerned, however, that this virus will undergo either adaptation or reassortment and lead to a severe pandemic. Thus far, human infections with the H5N1 avian influenza virus have been associated with an overall mortality of approximately 60%. Of the 433 cases reported to the World Health Organization through June of this year, 262 people had died.
A novel H1N1 influenza A virus containing genes from human, avian, and swine viruses was first identified in pigs in the United States in 1998. Although less significant than birds, pigs play an important role in the spread of influenza because they are susceptible to influenza virus from both birds and humans. Between 2005 and 2009, 11 cases of human infection with this triple-reassortment virus were described in the United States. In March and April of this year, further reassortment of this novel influenza A(H1N1) virus—one with uniquely different hemagglutinin and neuraminidase surface proteins—was identified in patients in Mexico. Transmissibility of the new H1N1 flu virus is high. Since initial cases of the novel H1N1 influenza virus were identified in Mexico, and then in Southern California, the virus has spread rapidly. In June, the WHO declared a pandemic. As of early September, tens of thousands of cases had been reported in the United States, and hundreds of thousands of cases had been reported worldwide.
It is important to appreciate the fact that pandemic influenza can occur in waves, with alternating periods of high infectivity and weeks or months of fewer infections; this pattern was particularly apparent in the 1918 pandemic.
In the 1918 pandemic, the second wave (lasting 8-10 weeks) occurred in the fall and was associated with a much higher mortality (up to 2%) than the first wave that had occurred in the spring. A third wave occurring in the spring of 1919 was similar to the first wave in terms of its high morbidity and relatively lower mortality.
Pandemics and Pregnancy
For reasons that are unclear, pregnant women have been observed to have higher morbidity and mortality compared with nonpregnant patients during influenza infections—seasonal or pandemic.
Observational reports of the 1918 pandemic paint a grim picture. One report published in the Journal of the American Medical Association in 1918, for instance, showed that 52 of 101 pregnant women who were admitted to Cook County Hospital in Chicago during a 2-month period with severe influenza succumbed to the illness. This mortality of 51% in pregnant patients was significantly higher than the observed 33% mortality rate in nonpregnant patients admitted to the hospital (719 of 2,154 nonpregnant patients who were admitted during the same time period died).
Additionally, among the 49 pregnant survivors in this sample, 43% either aborted or delivered prematurely (J. Am. Med. Assoc. 1918:71;1898-99). These are remarkable numbers.
Milder pandemics have had lower mortality overall, but reports have clearly shown that disproportionate numbers of pregnant women—particularly in the third trimester—have succumbed during influenza pandemics compared with the general population. An observational report from the milder 1968 pandemic, for instance, shows that pregnant women still were disproportionately represented among those dying during the pandemic.
Thus far in the current pandemic, the Centers for Disease Control and Prevention has reported similar trends—that pregnant women who contract the virus are significantly more likely to require hospitalization and are disproportionately represented among those who have died from it.
Of 34 cases of confirmed or probable H1N1 influenza in pregnant women that were reported to the CDC during the first month of the pandemic (mid-April to mid-May), 11 (32%) were admitted to the hospital. Dr. Denise Jamieson and her coinvestigators at the CDC noted that this hospitalization rate was four times higher than the hospitalization rate in the nonpregnant population due to influenza infection (Lancet 2009 Aug. 8;
This report by Dr. Jamieson also noted that the mortality is disproportionately elevated among pregnant women, especially in the third trimester. Four of six relatively healthy pregnant women who died during the first 2 months of the pandemic (mid-April to mid-June) were in the third trimester.