Tag Archives: swine flu

Swine Flu: New and Improved!

Contributed by guest blogger: Marni Hershbain ’11

Flu season is never enjoyable, but some seasons are certainly worse than others. The 2009 swine flu outbreak was particularly serious because the 2009 H1N1 strain was a novel virus, formed via the reassortment of swine, avian and human flu viruses. There were over 600,000 confirmed cases of H1N1 and over 18,449 deaths during the course of the pandemic. While this sounds pretty bad, it could have been much worse. The transmission efficiency of H1N1 was actually much lower than those of other pandemic strains, such as the 1918 H1N1 strain. Unfortunately, recent research demonstrates that this could change.

Flu strains are characterized by the hemagglutinin and neuraminidase found on their surfaces, hence names like H1N1. In order for the virus to infect a cell, hemagglutinin on the surface of the virus must bind to glycan receptors on the cell. Therefore, to explain the low transmission efficiency of 2009 H1N1, researchers looked to its hemagglutinin.
In most flu strains, the amino acids at positions 219 and 227 within the hemagglutinin are both hydrophobic or both charged. In 1918 H1N1 both are hydrophobic. However, the 2009 H1N1 strain has isolucine, a hydrophobic molecule, in position 219 and glutamic acid, a charged molecule, in position 227. Researchers hypothesized that lacking either hydrophobic or ionic interactions at these positions would disrupt the positioning of neighboring residues and decrease the hemagglutinin’s binding affinity. They further hypothesized that if they replaced isolucine with the charged amino acid lysine, stable inter-residue interactions would occur and binding affinity would increase.

When researchers compared the ability of wild type and isolucine→lysine mutant strains to bind to an array of glycans representing human binding sites, they found the binding ability of the mutant strain was 30 times greater. The mutant version also bound more intensely to receptors in human tracheal tissue. Researchers also infected ferrets (commonly used as models in human influenza studies) with either wild type or mutant virus. Only the ferrets infected with mutant virus spread the infection to all of the previously uninfected ferrets placed in close proximity to them.

The mutation of just one amino acid could greatly impact the transmission efficiency of 2009 H1N1. Flu viruses tend to mutate frequently, which is why a new vaccine needs to be developed every year. Predicting what these mutations will be is not an easy task, but mutations at the positions in this study will certainly be monitored closely.


The Role of Social Networks in H1N1 Transmission Within a School

Contributed by Guest Blogger: Aaron Grober ’11

The H1N1 subtype of the Influenza type A virus, known colloquially as “swine flu,” was the most common cause of human influenza infection in 2009, and remained a major concern in sparking a pandemic throughout the 2009/2010 flu season.

This recent paper examines the role of grade, class, and social network in transmission of this virus in a school setting. Taking a closer look at the actual transmission pattern of this novel subtype of influenza is critical in developing models to better predict and combat pandemic spread. In the case of this school, closure due to outbreak did not significantly affect transmission among students, indicating that it may have occurred too late to be effective, stressing the importance of more exact models. The study encompassed 370 students from 295 households, surrounding an H1N1 pandemic that occurred in a Pennsylvania elementary school in April and May 2009.

The researchers found that the structuring of the school into grades and classes significantly affected the probability of transmission: 3.5% between students within a class, five times less than that between students of the same grade but different class, and five times less than that between students of different grades.

The researchers took an in-depth look at fourth-graders. They note that children are four times more likely to play with members of the same sex, and found that this behavior had a significant impact on disease transmission; the onset of epidemic transmission occurred among boys significantly before that of female classmates. In addition, they found no significant difference between recorded playmate transmission rates, and the expected proportion for if being a playmate was not a risk factor. The researchers used class seating charts to determine if proximity to an infected individual affects the risk of transmission; as it turns out, they found that sitting next to an infected individual did not significantly affect one’s risk.

In addition to school structure, the researchers looked at spread within households. The probability of a child to adult transmission within a household depended significantly on the household size, where probability of spreading the disease is much lower in larger households than smaller ones. The predominant means of adult infection was from outside the home.

These unique findings shed light on the extremely complex transmission pattern within structured populations. The biggest factors for transmission within school are grade and class, but not seating arrangement, sex, but not playmate transmission. A number of obvious questions remain: Why does sharing a class, but not a desk-space affect transmission? Why is one more likely to transmit the disease in a smaller household than a larger one? This study is an extremely insightful epidemiological tool to help explain transmission, but our knowledge of how this virus spreads remains incomplete; it seems that the flu is far more complex than we imagined.


1918 and 2009

The 1918 influenza pandemic (the “Spanish Flu”), by some estimates, killed as many as 100 million people in a very short period of time.  The 2009 “Swine Flu” pandemic didnt kill so many, but it spread rapidly and widely across the globe.  Despite that difference, it turns out the two viruses responsible for these pandemics have some important similarities.

Influenza virus has a protein on the surface called hemagglutinin, or HA, which is used to attach to host cells, allowing the virus to then enter and replicate.  HAs change rapidly, which is partly why influenza keeps coming back.  When HA changes, your antibodies dont recognize it so well, so you get sick again.  It turns out that the HAs of 2009 and 1918 are similar on both the sequence and structure level.  There is a small patch on the HA protein that is 95% identical between 1918 and 2009 but only 70% identical to seasonal strains.  Looking only at the 3D structure, among all influenza HAs, the 2009 HA is most similar to the 1918 HA.  The 1918 and 2009 HAs also lack glycosylation at the tip, while seasonal influenza viruses HAs are sugary.

Why is that interesting? An unusual pattern was noted in the 2009 pandemic: elderly people were not as affected as younger people, the reverse of what is usually seen with influenza.  It was proposed that perhaps some people still had immunity to the 1918 virus, which continued to circulate for many years after 1918, and that immunity was cross-protective.  A recent study shows that this indeed seems to be the case.  Mice immunized with the 1918 virus are protected against the 2009 virus.  The converse is also true: if you immunize mice with the 2009 virus, they are protected against the 1918 virus.  That’s pretty impressive when you consider that one season’s vaccine might not protect you from next season’s virus. It seems the immune system cant really tell the difference between these viruses.  Note that it also tells us how long immunity can last!  The next question is, how and why has this HA structure come back?