Sunday, March 4, 2012

Understanding Vaccines and the HPV Mandate Debate



“Is not vaccination a substitute, necessarily futile, for faithful obedience to the sacred laws of health, whereby alone health can be conserved and disease prevented?”

Unknown Author, 1878


VACCINE HISTORY:

The origin of vaccinations goes back to over 200 years ago when the scientist, Edward Jenner, observed that British milkmaids who had been infected with the mild viral disease cowpox, were immune to being stricken with highly lethal smallpox virus. Jenner collected the pus from the milkmaid’s scabs and inoculated healthy people with this biological material, conferring to them immunity to smallpox. He called this material “vaccine” (from vacca, the Latin word for cow), and the practice of vaccination was born.


The British government, after seeing the effectiveness of Jenner’s vaccine, implemented laws requiring mandatory vaccinations of their citizens, expanding the government’s role in public health. Opposition to these mandates spread quickly and in some cases led to violent riots. Interestingly, many of the arguments of modern-day opponents of state-ordered vaccinations are rooted in those expressed back in 19th century Britain.


ACQUIRED IMMUNITY:

When our bodies are exposed to some foreign material, such as a virus, immune cells create protein particles known as antibodies that specifically stick on to the foreign target, tagging it to be eliminated by other immune system cells. The process of creating these specific antibodies is not immediate, and therefore it takes time for the immune system to recognize and eliminate the foreign target. Depending on the characteristics of the virus and the strength of the host’s immune system, infection may persist for days or even years.

If a host were presented with the same virus again, it would mount a rapid and effective immune response, stopping the virus in its tracks. This is because our immune system ‘remembers’ the virus by keeping the effective tagging proteins, or antibodies, ‘on-file’. Vaccines work by taking advantage of this ‘memory’ of the immune system.

While there are many types of vaccines, ranging from ‘dead’ viruses to a mixture of viral proteins, they all present a host’s immune system with sufficient information to create antibodies against the virus of choice. Vaccines act like the primary exposure to a virus without the risk of infection. When a vaccinated person is exposed to a ‘live’ virus later on, its immune system can eliminate the virus before it does any damage.


HERD IMMUNITY:

When a sufficient amount of a given population is vaccinated against a virus, those who have not yet developed immunity against the virus are still protected to a degree. This is known as the herd immunity effect. To understand how this works, imagine an individual infected with the seasonal flu who coughs onto someone who has had their seasonal flu shot. The individual coughed on will not contract the flu virus because he is immune. The vaccinated individual has protected himself from infection, but he has also prevented everyone who he comes into contact with later on from the flu virus as well. In this way, vaccination slows down the spread of an infectious disease and can even lead to its elimination.


DISEASE ELIMINATION:

A disease’s basic reproductive number, known as R0 (pronounced ‘R-naught’), determines the critical proportion of a population that needs to be vaccinated (pc) for a disease to be eliminated.

pc = 1 – 1/ R0

R0 depends upon the transmission likelihood (β), the contact rate (c) and the duration of infectiousness (D).

R0 = βcD


If a disease had an R0 value equal to 2, this would mean that on average one infected individual would cause 2 others to become infected. It would also mean that for the disease to be eventually eliminated at least 50% of the population would need to be vaccinated against it.


HPV VACCINE:

In June 2006, the Food and Drug Administration (FDA) of the United States approved the first vaccine against the Human Papillomavirus (HPV), a sexually transmitted disease. The approved Gardasil vaccine developed by Merck protects those vaccinated from the four strains of HPV associated with 70% of cervical cancer cases in women and 90% of genital warts cases. A Centers for Disease Control and Prevention (CDC) committee soon after added the Gardasil vaccine to the list of recommended immunizations for young girls aged 11-12.

In the wake of this CDC recommendation, individual state governments began to introduce various pieces of legislation to help fund this expensive ($360) three-dose vaccine and in some cases requiring the vaccination for school attendance. In February 2007, the governor of Texas, Rick Perry, issued an executive order mandating that girls entering the 6th grade be vaccinated with Gardasil. His decision caused enormous public backlash and the Texas legislature soon repealed the mandate.


HPV MANDATE DEBATE:

One reason for opposing this mandate could be that current voluntary vaccination rates of young women are sufficient to eliminate HPV. As of 2010, only 32% of 13-17 year olds in United States have completed the three-shot regimen. Since women comprise about half of the US population, this means that the total vaccination coverage of young adults is around 16%. Using conservative estimates of β = 0.6, c = 1.4, and D = 2, from the primary literature, R0 for HPV is equal to 1.68. This makes the critical vaccination threshold for elimination (pc) equal to around 40%. This would correspond to a critical vaccine coverage level of 80% if only young women are vaccinated. Therefore, current voluntary vaccination rates of young women against HPV are insufficient to eliminate the infection from the population. One might wonder then why there is so much controversy surrounding the HPV vaccine, Gardasil, in an era where mandatory routine vaccinations are prevalent.

As previously mentioned, opponents of mandatory vaccinations have been around for a long time. However, in modern-times, anti-vaccinationists are usually in the minority. In the current case of mandated HPV vaccination, though, opponents appear to be in the majority. What then is making the Gardasil vaccine such a controversial issue?

First of all, the Gardasil vaccine is relatively new to the market. While it has passed the rigors of FDA testing, the long-term effectiveness of the vaccine is unknown. Current studies show that the vaccine protects women for at least five years, but when most cervical cancer cases occur about 30 years after the recommended age of vaccination, the immunity to HPV might wear off.

Second, HPV infection is unlike many other communicable diseases in that its transmission is through sexual contact. While good hygiene may reduce one’s likelihood of contracting the seasonal flu, total isolation from others is the only 100% effective strategy. On the other hand, the contraction of HPV can be simply avoided by abstaining from sex. You can’t control if someone coughs on you, but you can control your sexual behavior.

Finally, mandating that only young women bear the risks and costs associated with HPV vaccination is unfair treatment. Men can contract HPV and get genital warts as well as a host of cancers, including penile cancer. While the Gardasil vaccine was originally targeted towards women, due to HPV’s close association with cervical cancer, solely mandating the vaccine for women is discriminatory. In a heterosexual population where most women are vaccinated with Gardasil, the men are effectively protected from HPV without having bore any of the cost.

Government mandated vaccines have caused the effective elimination of smallpox and polio among other diseases in the past. How the government should control the spread of HPV is still up to debate.

1 comment:

  1. I appreciated the big picture view presented by this blog (as well as the fantastic old political cartoon of the beast of vaccination!), since the history and science of vaccination provides the necessary context for understanding and evaluating the HPV vaccination debate.

    One point on which I'm a bit confused: in this blog, "herd immunity" is explained and then in the "disease elimination" section, the mathematics are provided that specific a threshold of vaccination at which the disease could potentially be eliminated. However, in the Javitt et al. (2008) article, it is stated that "There is no threshold value above which herd immunity exists, but as vaccination rates increase, indirect protection also increases also increases until the infection is eliminated." Maybe this statement is not contradictory to the blog's calculation of a threshold vaccination level at which the disease might be eliminated, but I don't quite understand how these two statements fit together.

    Prior to these blogs and the presentation, I had read newspaper articles about the HPV virus and its link to cervical cancer, and the case for mandated vaccination seemed obvious to me. And reading the ACIP recommendation prior to the presentation strengthened this sentiment.

    However, after reading Javitt et al. (2008), I began to question these convictions. In general, government mandates should be made with great care in a democratic society and as this article points out, mandated vaccinations are indeed rare and have been limited to those diseases which are highly contagious (small pox) or unavoidable (tetanus) and have rather immediate and high chances of causing death. While figures have been cited for the high infection rate for HPVs, the ratio of cancer-causing HPVs within that rate isn't clear and the authors of that article suggest its quite low. If HPV vaccinations are mandated, what other vaccinations could also be potentially mandated by the same arguments given that according to these authors, the criteria for mandating HPV vaccination would be historically novel compared to the criteria for previous vaccinations? The authors point out that mandates are typically tied to diseases which pose an overall public health threat as opposed to a threat at the personal level. In light of their articles, it seems that mandated vaccination for HPVs may not fit with historical criteria and perhaps a more appropriate solution would be more aggressive education about the threats and promotion of the vaccination such that voluntary vaccination would rise significantly. And subsidized/insurance-covered vaccinations would no doubt make a huge difference in the rate of voluntary vaccination.

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