Why no large outbreaks among adults?

High rates of absenteeism among essential workers has been identified as one of the greatest dangers of a pandemic. If there aren’t enough people to keep the heat on and the water flowing, there could be large numbers of deaths, regardless of the case fatality rate of the flu virus. The Southern Hemisphere countries faced the new H1N1 virus without a vaccine. Yet, there were no reports of large scale outbreaks among adults. As a consequence, no city in any country in the Southern Hemisphere experienced serious disruptions of essential services.

In the US, although large scale outbreaks in schools are being observed with increasing frequency, we are not seeing similar outbreaks among adults. In a study done in Chicago early in the pandemic, the attack rate in different ages was quantified.

From Ritger et al. (2009) 2009 Pandemic Influenza A (H1N1) Virus Infections — Chicago, Illinois, April–July 2009. MMWR Weekly, August 28, 2009

From April 24 to July 25, a total of 1,557 laboratory-confirmed 2009 pandemic influenza A (H1N1) virus infection cases among Chicago residents were reported to CDPH with specimen collection dates of April 23 to July 16 (Figure 1). Although an initial cluster was identified in one northeastern community area during the first week of the outbreak, cases soon were reported among residents of multiple community areas throughout the city (Figure 2). By May 23, the fifth week of the outbreak, cases had been reported in 68 of Chicago’s 77 community areas.

During April 24–July 25, the median age of reported confirmed cases was 12 years (range: 24 days–91 years). The attack rate was highest among children aged 5–14 years (147 per 100,000 population), followed by children aged 0–4 years (113 per 100,000). The attack rate for children aged 5–14 years was 14 times higher than for adults aged ≥60 years (Table).

The table provides information on the relative number of cases in each age group:

  • 0-4 – 16%
  • 5-14 – 40%
  • 15-29 – 22%
  • 30-59 – 18%
  • over 60 – 3%

Although this information is from a study conducted early in the pandemic, these percentages roughly correspond with media reports today. The overwhelming number of people being infected are children. The elderly are being infected in very low numbers. Why might this be?

One possibility is that older adults may possess some immunity against the new H1N1 virus. One study suggested that this might be the case (Katz et al. 2009), but another found no evidence for such pre-existing antibodies except in the very old (Itoh et al. 2009). Given the conflicting results, the jury is still out on this hypothesis.

A second possibility is that the conditions for large outbreaks among adults have not yet been met. Seasonal flu does not peak in the US until January or February, in most years. Perhaps we will see large scale outbreaks in adults at that time. However, large scale outbreaks in adults did not occur in the Southern Hemisphere, even in their winter.

A third possibility is that interrupting chains of transmission with school closures and extensive distribution of Tamiflu may be protecting older adults. Influenza normally spreads among children first, then their parents and middle-aged adults, and finally the elderly. Interventions triggered by outbreaks in children may be limiting the spread of the virus to older groups.

A fourth possibility is that the virus is still adapting to humans and has yet to adapt sufficiently so as to be transmitted efficiently from one adult to another.

These possibilities are not mutually exclusive and may all play a role in the relatively low attack rate among adults. As the virus continues to spread in the Northern Hemisphere, we should have more data with which to decide among these possibilities.

The issue of what is causing the low attack rate among adults and whether this will change deserves more attention than it has received thus far, in my opinion. If we suddenly start seeing large outbreaks in adults, we will see many more deaths (because the case fatality rate is higher in adults) and we may see break down of essential services. And that may be the greatest danger of all.


Itoh et al. (2009) In vitro and in vivo characterization of new swine-origin H1N1 influenza viruses. Nature, 460: 1021-1025.

Katz et al. (2009) Serum Cross-Reactive Antibody Response to a Novel Influenza A (H1N1) Virus After Vaccination with Seasonal Influenza Vaccine. MMWR, 58: 521-524


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