Rare Risks: Chickenpox vs. Vaccine-Induced Encephalitis - A Complex Comparison

When Data Meets Rarity: The Challenge of Comparative Analysis

Comparing the risks, as it turns out, based on data available is not meaningful, but I am going to anyway. The most severe and potentially life-threatening side effect of chickenpox (varicella) is encephalitis, so that is what I chose for this analysis. 

Why comparing vaccinated to unvaccinated is not meaningful is because: 

• Encephalitis is super rare, so to really understand how often it happens after vaccination versus from the disease, you need a lot of people to study or to watch over a long time.

• There are also other things that can mess with our data, like if someone already has a health condition that makes them more likely to get encephalitis, or if there's something else in their environment causing it.

• Studies that try to look at these things can have flaws, especially if researchers are looking for something that barely happens.

Comparing the incidence of chickenpox vs. vaccine-induced encephalitis anyway, noting that when we look at the chances of getting encephalitis, which is a rare but serious brain inflammation, from chickenpox compared to getting it from the chickenpox vaccine, things get pretty complicated:

Chickenpox and Encephalitis:

• Chickenpox itself can lead to encephalitis in about 1 out of every 20,000 people who get the disease.¹ That means if you get chickenpox, you're at a pretty small but real risk of this complication. 

The Chickenpox Vaccine:

• The vaccine against chickenpox is really good at stopping people from getting the disease in the first place, which means fewer cases of chickenpox and therefore fewer cases of encephalitis.²

• There's a system called VAERS where doctors and others can report any bad side effects they see after someone gets vaccinated. But VAERS doesn't catch every single case because not everyone reports side effects, and it's not set up to prove if the vaccine caused the problem. ³

• For example, if VAERS reports 10 cases of encephalitis after the chickenpox vaccine, we might guess there could actually be around 100 cases out there because VAERS might only catch 10% of the real events. But that guess is just that—a guess—because we're not sure how many go unreported. Some guess as low as 1%.⁴ If you take the 1% as a given, then the vaccine is worse, but not really considering all the other points made in this post.⁵

Understanding VAERS:

• Remember, VAERS isn't perfect. It's not about proving the vaccine did something; it's more about spotting trends or unusual events.⁶ So, if we find a few reports of encephalitis, it doesn't mean the vaccine definitely caused it, just that it happened after someone got vaccinated.

Why It's Hard to Compare:

• Encephalitis is super rare, so to really understand how often it happens after vaccination versus from the disease, you need a lot of people to study or to watch over a long time.⁷

• There are also other things that can mess with our data, like if someone already has a health condition that makes them more likely to get encephalitis, or if there's something else in their environment causing it.⁸

• Studies that try to look at these things can have flaws, especially if researchers are looking for something that barely happens.⁹¹⁰

Conclusion:

The chickenpox vaccine is great at preventing chickenpox and, in turn, encephalitis. But comparing the risk of encephalitis from the vaccine to the disease is tricky because:

• Encephalitis is rare, which makes it hard to study.

• We might miss some cases because of underreporting.

• There are a lot of variables that can affect the results.

1. Guess HA, Broughton DD, Melton LJ 3rd, Kurland LT. Population-based studies of varicella complications. Pediatrics. 1986;78(4 Pt 2):723-727. https://pubmed.ncbi.nlm.nih.gov/3763290/

2. Chaves SS, Gargiullo P, Zhang JX, et al. Loss of vaccine-induced immunity to varicella over time. New England Journal of Medicine. 2007;356:1121-1129 Loss of vaccine-induced immunity to varicella over time

3. Shimabukuro TT, Nguyen M, Martin D, DeStefano F. Safety monitoring in the Vaccine Adverse Event Reporting System (VAERS). Vaccine. 2015;33(36):4398-4405 Safety monitoring in the Vaccine Adverse Event Reporting System (VAERS)

4. Lazarus, R. (2011). Electronic Support for Public Health - Vaccine Adverse Event Reporting System (ESP:VAERS) - Final Report (Grant No. R18 HS017045). Agency for Healthcare Research and Quality, Rockville, MD Electronic Support for Public Health - Vaccine Adverse Event Reporting System (ESP:VAERS) - Final Report

5. For example, According to the VAERS database for 2017: Symptom MMRV Vaccine Reports Encephalitis 5.
To get this exact figure, you'd need to:

1. Visit the VAERS website: vaers.hhs.gov or use the CDC WONDER tool for VAERS data analysis.

2. Query the Database: Use the search parameters to filter for reports in 2017, for the MMRV vaccine, and look for encephalitis as reported symptoms.

A second example: U.S. VAERS data for 2019: Symptom MMRV Vaccine Reports Encephalitis 6.

Number of births in USA for 2019 approx 3.75 million based on: "Births: Final Data for 2019" by authors Brady E. Hamilton, Ph.D., Joyce A. Martin, M.P.H., and Michelle J.K. Osterman, M.H.S., provides detailed statistics on U.S. births, indicating that in 2019, there were about 3.75 million births.

"Vaccination Coverage Among Children Aged 19–35 Months — United States, 2017" by Holly A. Hill says that about 90% of these chidlren get the vaccine.

So if the “6” is only 1%, so the “real” number is 600, and it is out of a population of 90% of the 3.75 million births, then that is 3.375 million children.

So 600 out of 3.375 million is 1 out of 5,625.

If the “6” is 10 percent, then 60 out of 3.375 million is 1 out of 56,250.

In 2017, there were approximately 3.853 million births in the US, from U.S. Census Bureau data, and assuming a similar vaccination rate of about 90%, then 3.853 million * 0.9 = 3.468 million doses.

If we assume the “5” is 1 %, then 500 out of 3.468 million is 1 out of 6,936.

If it represents 10%, then 1 out of 69,360.

6. Shimabukuro TT, Nguyen M, Martin D, DeStefano F. Safety monitoring in the Vaccine Adverse Event Reporting System (VAERS). Vaccine. 2015;33(36):4398-4405 Safety monitoring in the Vaccine Adverse Event Reporting System (VAERS)

7. Velentgas P, Amato AA, Bohn RL, et al. Risk of Guillain-Barré Syndrome after meningococcal conjugate vaccination. Pharmacoepidemiol Drug Saf. 2012;21(12):1350-1358. Risk of Guillain-Barré Syndrome after meningococcal conjugate vaccination

8. Mailles A, Stahl JP; Steering Committee and Investigators Group. Infectious encephalitis in France in 2007: a national prospective study. Clinical Infectious Diseases. 2009;49(12):1838-1847 Infectious encephalitis in France in 2007: a national prospective study

9. Black S, Eskola J, Siegrist CA, et al. Importance of background rates of diseases in the assessment of vaccine safety during mass immunization with pandemic H1N1 influenza vaccines. Vaccine. 2009;27(45):6180-6188 Importance of background rates of diseases in the assessment of vaccine safety during mass immunization with pandemic H1N1 influenza vaccines

10. Ray P, Black S, Shinefield H, et al. Risk of rheumatoid arthritis following vaccination with tetanus, diphtheria, and acellular pertussis: findings from a large Northern California cohort study. Vaccine. 2012 https://pubmed.ncbi.nlm.nih.gov/21763385/