Before I get into the vaccine and the article, I'd like to give a brief overview of what norovirus is and why it merits research. Norovirus is a gastrointestinal virus (virus that infects the gut) that causes vomiting and diarrhea. In severe cases, infected people (usually infants/toddlers or the elderly) can become dehydrated and sometimes die. While norovirus only kills about 200 people per year in the United States, there are over 200,000 deaths per year worldwide in children under five alone. In addition, over 21 million people in the U.S. become infected with norovirus (the article says 5.5 million, but this is not the correct CDC estimate--they may be citing confirmed sequenced cases or something); worldwide infection rates are around 267 million/year. While Salmonella and E. coli traditionally get most of the press as food poisoning culprits, norovirus actually causes more food poisoning than either of these bacteria.
Norovirus is typically passed via the "fecal-oral" route, which means that the virus is found in feces and is passed when fecal particles get on a person's hands or on surfaces that people touch and then these people touch either food or put their hands/fingers in their mouths and become infected. Norovirus outbreaks typically occur on cruise ships and in schools, nursing homes, universities, military barracks, restaurants, and other places where people are in close proximity. An outbreak of norovirus is hard to control because this is a very hardy virus with a very low infectious dose. It takes less than 10 particles to become infected! Even if you do a good job of washing your hands, 10 particles can easily be left over and seed an infection. In addition, alcohol-based hand sanitizers (like Purell) don't work well against norovirus, and their use has actually been linked to increased outbreaks in a hospital setting. On top of this, people who are infected "shed" virus for several days (and sometimes weeks) after they recover from the illness and therefore may be contagious long after the diarrhea and vomiting is over.
There are several reasons that development of a vaccine for norovirus is warranted. 1) Norovirus outbreaks are extremely costly in terms of productivity and economics. Schools shut down, hospital wards shut down, military operations shut down, cruises return to port, etc... Studies have found that a single outbreak of norovirus in a hospital setting can cost hundreds of thousands of dollars. 2) This virus infects hundreds of millions of people every year and has a fairly high worldwide death toll. 3) Because of the low infectious dose and hardiness of norovirus, it is a potential bioterrorism agent. 4) NO ONE (well, I might be the exception) likes to get norovirus. Who wants 2 days of explosive diarrhea and vomiting? Wouldn't you rather get a nasal vaccination?
Now, getting on to the article. This particular vaccine is made from virus-like particles (VLPs). Norovirus VLPs have the outside (capsid) of the virus but do not contain any of the internal genetic information, so they cannot reproduce (replicate). There is no danger of acquiring a norovirus infection from a VLP vaccine. The reason that the VLPs protect a person from an actual norovirus infection is that they contain the part of the virus that immune cells recognize, so the body can produce antibodies against them. To give you an idea of what norovirus (or at least the VLP) looks like, here's an electron microscopy picture of a norovirus VLP that I made in lab. This study is called a human challenge study. The basic idea is that the study participants are given a vaccine and then infected with the pathogen to see if they are protected. The researchers had 77 participants who completed the study. Thirty-eight of these people were given the vaccine and 39 were given a placebo (they were given a nasal treatment that did not contain any VLPs). The study participants do not know which group they are in. The vaccine treatment contained VLP, an adjuvant (a substance that makes the immune response stronger), chitosan (a substance that makes the vaccine stick to the inside of your nose so it doesn't get washed away before it has a chance to work), and sucrose and mannitol (to stabilize the VLPs). The placebo treatment only contained the sucrose and mannitol. When conducting research on any drug, researchers need a placebo group because this allows researchers to determine if there are actually any good or bad effects from the drug or whether reported side effects and benefits are just psychological. For instance, stuffy nose was one of the reported symptoms, but this occurred equally in both the vaccine and placebo groups. This shows that this particular symptom is probably not related to the active part of the vaccine. (This case is most likely due to the nasal delivery of the vaccine.)
The study participants were given an initial dose of the vaccine or placebo, and then given a "boost" (a second dose) three weeks later. The timing is important because it takes the body several weeks to develop "protective immunity". Protective immunity means that your body has produced antibodies that are capable of fighting off a future infection. The boost is given because it makes the immune system's protective response stronger. Three weeks after the second dose of vaccine or placebo, the study participants were infected with the same strain of norovirus they were vaccinated against to see if they would get sick. If the vaccine works, we expect that significantly more people in the placebo group will get sick than in the vaccinated group. The study participants were monitored for at least 4 days for gastrointestinal illness.
The results showed that the vaccine gave some protection from norovirus infection. Sixty-nine percent of the placebo group got sick (these are the ones who were not vaccinated), while 37% of the people who were vaccinated got sick. Overall, this shows a relative reduction in gastrointestinal illness of 47% (about 50%) in vaccinated people. The msnbc article is a little misleading in how they present the protection. They indicate that the vaccine is about 60% effective. I think they got to this number because only 37% of the vaccinated group got sick (leaving 63% that did not get sick). This isn't quite correct because not all of the placebo group became ill, so not all of the protection was due to the vaccine. The relative reduction that they give in the paper (47%) is a more accurate representation of the actual protection because it takes this into account.
Now I'd like to tell you why I'm not as impressed with these results as the headline "Norovirus Vaccine Shows Promise" would like me to be. I would like to applaud the authors of this study because they do a good job of pointing out the drawbacks of their findings in the published paper. However, the drawbacks are not conveyed in the news article communication to the general public, which I think is a problem. 1) This was done with one type of norovirus, a strain we call GI.1 noroviruses (or Norwalk virus). This group of noroviruses is relatively stable--they don't mutate as much as the other group of noroviruses that infect humans. We might expect that a protective response would be easiest to get from this strain since it doesn't change much. However, GI.1 noroviruses only make up around 10% of infections. Most human infections (about 70-80%) are caused by GII.4 noroviruses, which mutate a lot more. It might be harder to get a long-term protective immune response from a GII.4 VLP vaccine. 2) They infected the participants with the exact same strain they gave them the vaccine against. This is common practice for an initial study like this, which is fine. But there may be several circulating strains within a year between the GI and GII noroviruses. A vaccine with a mix of different strains is a must. While they do address this in the article and in the paper, the study that was done was only done with one strain. We do not yet know how having multiple strains will affect the immune response in humans. It will likely still be protective, but I am anxious to see some results from a multiple-strain (multivalent) vaccine in humans. 3) This is a big one. They did their infection with virus only three weeks after the second vaccine dose. All we can tell from this study is that a VLP vaccine protects for up to one month. 50% protection after one month is really not that great. What we really need to see is results from an infection done 6 months or a year after vaccination. I know they are working on this, but it is important to understand that these results are only "promising" if protection lasts for at least a year (or a disease season). No one is going to get re-vaccinated every few months, and a pharmaceutical company would probably not waste money backing a vaccine that has to be administered more than once per year. I think the bottom line is that when they say "If all goes well, a norovirus vaccine could come to market in the next four to five years," it is actually very unlikely that will happen based on these results.
I've been a little negative here, but I am not saying that developing a viable VLP norovirus vaccine is impossible (and of the different norovirus vaccine options, it is the most promising so far). There are many options they could pursue in order to improve the vaccine, which I am sure they are already doing. They could try different adjuvants to determine if there is one that can better stimulate the immune system. They could try producing the VLPs in a different way. They could try to increase the vaccine dosage. It is especially challenging because human challenge studies are expensive and take years to get permission to do. Because we have not yet found a way to grow human norovirus in lab-cultured cells or in mice, there is a lot we do not know about it. Designing a norovirus vaccine is a tricky task, and while we may not be there yet, I am confident we will get there.
well written! interesting. what's next????
ReplyDeleteAgreed!
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