The CDC’s Emerging Infections Diseases journal will publish in April a study (PDF file) examining household transmission of H1N1.

The investigators tracked patients in San Antonio, TX, (in April and May of 2009) who had lab-confirmed H1N1. They also recorded any “secondary case-patients” among housemates of the flu sufferers — i.e., people who were (presumably) sickened by their H1N1-infected housemates.

The study concludes that children (people under 18) were “disproportionately affected” by H1N1:

The highest proportion of laboratory-confirmed pandemic (H1N1) 2009 and secondary attack rates occurred in children, a finding consistent with the epidemiology of seasonal and pandemic influenza, where we know children experience higher rates of illness [...] and higher secondary attack rates [...]

It also determines that the “secondary attack rate” (contagiousness) appeared to be lower than for seasonal flu:

The secondary attack rate was 4% for laboratory-confirmed pandemic (H1N1) 2009, 9% for ILI [influenza-like illness], and 13% for ARI [acute respiratory infection]. In general, these rates are lower than for seasonal influenza and lower than anticipated for a pandemic strain [...].

You can read up on the full scope and “several limitations” of the study here (PDF file).

A study published today in the Journal of the American Medical Association suggests that vaccinating kids against H1N1 helps protect their entire community.

The researchers set out to test the idea of herd immunity and the observation that “[c]hildren and adolescents appear to play an important role in the transmission of influenza.”

The randomized, blinded study examined 46 volunteer Canadian Hutterite colonies. Roughly 80% of children aged 3 to 15 in half the colonies were given an H1N1 vaccine. Roughly 80% of those in the other colonies were given a hepatitis A vaccine.

Comparing the two populations, the study determined that vaccinating children against H1N1 “conferred 61% indirect protection against influenza among persons who did not receive the study vaccine.” The overall conclusion:

Our data suggest that a significant herd immunity effect can be achieved when the uptake of vaccine is approximately 80% in clusters in which children and adolescents aged 3 to 15 years are immunized.

The authors also propose that elderly people may benefit more from child-based herd-immunity than from being vaccinated themselves:

Although there were relatively few elderly individuals in this population, the protective effect is likely comparable with or greater than what can be achieved by direct immunization.

Helen Branswell, medical reporter for The Canadian Press, notes that children may not necessarily be the key to herd immunity:

Dr. Allison McGeer, an influenza expert at Toronto’s Mount Sinai Hospital, questioned whether the effect was due to the fact that Loeb’s team vaccinated kids, or just to the fact that they vaccinated a sizable portion of the population of the colonies randomized to get flu shots.

The New York Times piece about the study is here. The National Institute of Allergy and Infectious Diseases press release is here.

This CDC chart summarizes the data:

[CDC]

Click here to find the data broken down into helpful bar graphs. Also to learn how CDC compiles its estimates.

We blogged recently about a Pittsburgh-area study that suggests roughly 63 million Americans were infected with H1N1 in 2009. This estimate — although based on a methodology completely different from the CDC’s — falls in the mid-range of the CDC estimates. (Note: the CDC numbers include data for two extra weeks in January 2010.)

A study (PDF file) released in February by the Institute for Women’s Policy Research suggests that the lack of paid sick days in the private sector increased the spread of H1N1.

Some of the study’s interesting findings:

• “The vast majority of public sector employees receive paid sick days, but two of five private sector employees have no access to paid sick days.”
• “[E]mployees who attended work while infected with H1N1 are estimated to have caused the infection of as many as 7 million co-workers.”
• “The data suggest that more than 90 percent of public sector employees, but only 66 percent of private sector employees, took time away from work when infected with H1N1 [...] implying that many more private sector employees felt that it was necessary to attend work while ill.”
• “[T]he drop in absence rates between October and November was twice as steep in the public sector as it was in the private sector, suggesting that contagion was less common in the public sector.”

The study goes on to propose that “similar patterns of absence” might be “found among children and students” depending on whether their parents “have access to paid sick days to care for family members.”

(Our usual disclaimer: we can’t vouch for the study’s methodology or results; we offer it up as something you might be interested in reporting on.)

[Update 4 March 2010: This is an issue that public-health experts have worried about throughout the H1N1 pandemic. New York Times article from November 2009 here and WNYC report from September 2009 here.]

Two different assessments of U.S. immunity to H1N1 have emerged recently. The first suggests the number of Americans infected by swine flu in 2009 (roughly 63 million). The second estimates the number of Americans who currently have immunity to H1N1 (somewhere roughly between 150 and 165 million).

Ted Ross [U. of Pittsburgh]

Assessment #1: A new study published in PLoS Currents: Influenza estimates the number of Americans who were infected with H1N1 in 2009. (Read our post about PLoS — the Public Library of Science — here.) The lead author is Ted Ross, an associate professor of microbiology and molecular genetics at the University of Pittsburgh.

The study looks at levels of antibodies to 2009 H1N1 in Pittsburgh-area residents. (Infection with a virus stimulates antibody production, which then confers immunity.) It examines blood from “846 persons that ranged in age from 1 month to 90 years of age.” The samples were taken from “hospital and clinic patients in mid-November and early December 2009.”

It’s possible that some people with antibodies to 2009 H1N1 got them from the vaccine rather than infection with the virus — but “the timing of the sampling relative to vaccine availability in Pittsburgh suggests that these samples are likely from a largely unvaccinated population during the peak of the second pandemic wave.” In other words: the data probably approximate the number of people actually infected by H1N1 in 2009.

Here’s the study conclusion in a nutshell:

21% of persons in the Pittsburgh area had become infected and developed immunity. Extrapolating to the entire US population, we estimate that at least 63 million persons became infected in 2009. As was observed among clinical cases, this sero-epidemiological study revealed highest infection rates among school-age children.

Ian York [Michigan State University]

Assessment #2: Ian York, an assistant professor of microbiology and molecular genetics at Michigan State University, recently offered his best educated guess of the number of Americans now immune to H1N1 (on his blog Mystery Rays from Outer Space). As he puts it, there are three ways in which someone could have acquired immunity:

They could have been exposed to a related virus, some time in the past, and have developed a long-term immunity. They could have been infected with [H1N1], somewhere in the first or second wave. Or, of course, they could have been vaccinated.

He collects the available data for each of those categories — emphasizing that “[t]hey’re more or less approximate” — and concludes that:

• Over half the US population as a whole is now immune to the new [H1N1].
• As many as three-quarters of the elderly and two-thirds of the children — the most important populations as far as flu is concerned — may be immune.
• Between a third and about half of this immunity was due to vaccination.

To find York’s full table of high and low estimates — broken down by age group — click here.

Why is it useful to know what percentage of the population may have immunity to H1N1? As the PLoS study puts it, it “provides valuable information about the likelihood of a possible third wave and may be useful in decision-making about immunization strategies.” Or as York writes, the “level of immunity” that he calculated “is probably enough to impact virus transmission drastically.”

The CDC’s new H1N1 data re-emphasize that, compared to seasonal flu, swine flu has disproportionately affected people under 65. Vincent Racaniello, a professor of microbiology at Columbia University, recently summarized a study done in mice that suggests one possible reason for this. The study appears to confirm initial hunches that many older people have immunity to swine flu because of previous H1N1 vaccines or bouts of flu.

The mice, Racaniello says, were inoculated with various past strains of H1N1 — and it turns out that several of those strains offered significant protection:

In other words, if you lived before 1943, or received the 1976 swine flu vaccine, you may be protected against infection with 2009 H1N1 virus. After the 1976 swine H1N1 outbreak at Fort Dix, NJ, approximately 40 million people in the United States were immunized with an NJ/76 vaccine. The NJ/76 swine virus never spread in the general population, but the vaccine against it has finally proven useful.

If you are less than 35 years old, you are more likely to be infected with the 2009 H1N1 virus because you did not receive the NJ/76 vaccine, nor were you infected with viruses that circulated from 1918-1943.

Racaniello blogs regularly at Virology Blog. You can find his previous posts exploring the same topic here, here, and here. The mouse study that Racaniello cites is here.

The CDC put out some new H1N1 numbers on Friday: on cases, hospitalizations, and deaths. The data span the period from April 2009 to 16 January 2010. The CDC emphasizes that its numbers are estimates based on a “nationwide surveillance system” and “statistical modeling.” It’s impossible to generate exact numbers for several reasons:

[M]any people with flu don’t seek medical care and only a small number of those that do seek care are tested. More people who are hospitalized or die of flu-related causes are tested and reported, but under-reporting of hospitalizations and deaths occurs as well.”

With those caveats, here are the estimated numbers:

CDC estimates that between 41 million and 84 million cases of 2009 H1N1 occurred between April 2009 and January 16, 2010. The mid-level in this range is about 57 million people infected with 2009 H1N1.

CDC estimates that between about 183,000 and 378,000 H1N1-related hospitalizations occurred between April 2009 and January 16, 2010. The mid-level in this range is about 257,000 2009 H1N1-related hospitalizations.

CDC estimates that between about 8,330 and 17,160 2009 H1N1-related deaths occurred between April 2009 and January 16, 2010. The mid-level in this range is about 11,690 2009 H1N1-related deaths.

To find a more detailed chart showing the numbers for three different age ranges (0-17 years, 18-64 years, 65 years and older), click here. This chart is especially interesting because it illustrates how strongly h1n1 has affected young people. The same page also explains the methodology CDC uses to generate its H1N1 data.

And by a clear margin, swine flu is the winner and champion. Last April, when the outbreak first appeared in Mexico and the United States, “swine flu” dominated the public consciousness, news reports, and our Google searches. The huge volume of these early searches gives “swine flu” a substantial victory in total number of searches over time.

But by the chart below, you can see that “H1N1” picked up the pace and essentially pulled even once the initial wave of “swine flu” searches died down. And in recent months, as the second wave of the pandemic lessened, “H1N1″ is occasionally the more popular search term.

There are also geographic differences in the popularity of the two search terms. Below are maps for “swine flu” in red and “H1N1″ in blue. The darker the color, the more common the search. Between April 09 and January 2010, West Virgina, Utah and Maine produced the most searches for “swine flu”. During the same period, the upper Midwest led the way with the most “H1N1” searches.

Another interesting set of data to look at is the differences in related search terms. “Swine flu” searches have been more often associated to finding information about the symptoms. “H1N1″ searches, on the other hand, are more likely to be about vaccine information.

This kind of search data can provide an interesting window into both the rhetorical trends of media and how we talk about the pandemic. Perhaps we tend to use “H1N1” in more medical and scientific contexts –- the research, the pandemic data, and the vaccine. In contrast, “swine flu” may have been the choice in describing the illness, its traits, and the experiences of those who caught the flu.

You can do your own search term analysis at Google Insights for Search.

Here’s the short course: The vaccine is now available to pretty much everyone who wants it all across the country. So far, the US hasn’t decided to return/donate/sell any of its vaccine supply as some European countries are doing. Instead, it’s focusing on the upcoming (10-16 January) National Influenza Vaccine Week (NIVW). NIVW is another big push to encourage Americans to get H1N1 and seasonal-flu shots. It has its own schedule of activities, informational materials, a media toolkit, and web tools like ecards and badges.

Why continue pushing the H1N1 vaccine when, as Schuchat announced, “we’re seeing drops in laboratory confirmed hospitalizations and deaths”? Because there’s also “activity increasing in a few other indicators.” She clarified:

We still have more activity than we usually have this time of year, though it’s certainly much below where it was several weeks ago. All the virus that we’re seeing right now is the H1N1 virus. We haven’t yet seen the emergence of seasonal flu strains in any numbers at all. We saw a slight uptick in the last week’s reporting in the influenza-like illness visits to the doctors or emergency departments. That can sometimes happen right around Christmas, so we don’t know if that will persist. We also saw an uptick in pneumonia or influenza deaths in this past week. And that isn’t something that we necessarily see around the Christmas holiday.

Minnesota was one state that reported increased influenza-like illness last week.

What CDC is afraid of, of course, is a third wave of H1N1. To drive the point home, Schuchat showed this graph of pandemic mortality in 1957:

[CDC / usable on your site]

She went on to say:

This is really a reminder of why we are saying that we need to remain vigilant. [...] [I]f you look at this graph, the bottom part of that curve, you know, the — there’s the camel hump and then it comes down to that valley. Well, that’s where we are right now in that valley. We don’t know what’s going to happen over the next several weeks or months. But in 1957, this essentially gave the all clear whistle in that December/January time period. They had vaccine, but they didn’t encourage its use and yet they did go on to see that increase in mortality.

National Influenza Vaccine Week launches on Sunday. You can check here for NIVW activities in your area.

Two polls released just before the holidays give some new figures on H1N1 vaccination rates in the US: one by the Harvard School of Public Health (HSPH) and one by the National H1N1 Flu Survey (NHFS). The HSPH poll was conducted on 16-17 December, 2009. The NHFS poll was conducted on 6-12 December, 2009.

The HSPH results include the following:

The NHFS results include these numbers: