written by Tara Haelle

When parents have questions or concerns about vaccines, they are often seeking certainty, even if they do not consciously recognize that is what they want. They want a guarantee that the vaccines they give their children are completely safe and effective, a completely understandable desire – but one which science can never deliver.

Neither can science deliver certainty that their child will not catch a disease and suffer serious consequences, or death, from it. Science is in the business of uncertainty, but in that sea of uncertainty it offers solace in the consensus, the coming together of study after study after study which, taken together, can provide the fullest picture possible of a particular issue.

When it comes to vaccines, we are fortunate to have so many thousands of studies that the consensus is overwhelming. It is impossible to have a single massive study showing the safety and effectiveness of vaccines, but it’s also undesirable. Strength is in numbers, in the many studies that investigate all the little details related to vaccines so that we have that giant picture.

Yet what I love about science and research is that it never stops. Because we can never have 100% certainty, we never stop seeking more information, or, at the least, more confirmation of what we already know. So I was thrilled to see another systematic review from Pediatrics which reviews all the safety evidence for every vaccine on the CDC recommended childhood immunization schedule.

Systematic reviews bring together all the most current evidence and assess it together, and this one uses data from 67 studies to seek out everything we know about possible side effects from recommended vaccines. It essentially updates the findings published by the independent Institute of Medicine in 2011 on Adverse Effects of Vaccines, adding in information published since then and about four vaccines not included in that review.

The authors – who have no pharma ties – searched eight medical research databases, the IOM report, vaccine package inserts, review articles’ bibliographies and statements from the FDA and the CDC’s Advisory Committee on Immunization Practices to locate more than 20,000 possible studies.

From these, they narrowed it down to 67 papers by selecting only studies that actively looked for possible adverse events involving currently recommended childhood vaccines available in the US. All included studies needed a control group or to compare vaccinated children to unvaccinated children, and no animal studies were included. The data from the IOM report was considered along with the new data in these 67 studies.

Conveniently, they organized the findings by vaccines in alphabetical order, and so I’ll do the same in reviewing what they found. Their only goal was to look for serious adverse events that were likely caused by a vaccine. Therefore, on the one hand, this could be a frightening blog post – it literally lists every possible thing that evidence has shown could go wrong after vaccines. On the other hand, seeing all the worst case scenarios and seeing what’s NOT on this list should be encouraging.

However, the researchers did *not* usually report on the rates of these adverse events, only on the possibility that they could occur. Some of these adverse events occur to 1 in 1,000 children. Others occur only to 1 in several millions or only to those who are immunocompromised and are therefore not supposed to receive that vaccine anyway (such as contraindicated conditions for the MMR, which is a live vaccine). I have added after each paragraph what rate is reported by the CDC for each adverse event described here, when available. Those rates are based on what has been found in research studies as well.

Heads up: This reads like a dull laundry list of sorts; it’s pretty dry reading. I didn’t want to jazz it up at the expense clarity or accuracy.

The researchers found no evidence that the DTaP (diphtheria, tetanus, acellular pertussis) vaccine causes any conditions, including diabetes (which four studies specifically looked for), or serious adverse events.

(Note: The CDC lists high fevers (up to 105 F among 1 in 16,000 recipients) and febrile seizures (1 in 14,000) as a possible adverse event, but the actual evidence is mixed on that. The previous whole cell DTP vaccine definitely could cause febrile seizures, but research on the acellular pertussis vaccine has found either no association with seizures or a much lower rate, as little as 1 in 19,500 children. Febrile seizures are already not uncommon among children, with an estimated 1 in 25 children experiencing them at some point in childhood. They do not cause brain damage, learning problems or epilepsy.)

The Hib (Haemophilus influenzae type B) vaccine was not included in the IOM report. The researchers found it could cause redness and swelling at the site of the injection but did not lead to hospitalizations or high fevers. The Hib vaccine had no link to any serious adverse events, including convulsions, diarrhea, fungal infections or reflux disease.

The Hepatitis A vaccine was also not included in the IOM report, and there were no studies that looked for adverse events with the hepatitis A vaccine by itself. However, a study that looked at vaccines together found that the hepatitis A vaccine was possibly linked to purpura, or purple spots under the skin, in children aged 7 to 17.

The Hepatitis B vaccine may cause an allergic reaction in those allergic to yeast. The IOM did not find enough evidence to identify any short-term or long-term serious adverse events in children and found the vaccine did not cause multiple sclerosis. The CDC estimates allergic reactions may occur in 1 of every 1.1 million doses.

The inactivated polio virus vaccine was not included in the IOM study. The researchers found that newborns immunized against polio may have a higher risk of food allergy sensitivity, but the evidence was very weak and contradicted by another study not finding a link.

The flu vaccine – live or inactivated – was not found to cause any serious adverse events by the IOM, and new research since then supports that finding. However, observational studies found a small risk of febrile seizures and short-term gastrointestinal symptoms (vomiting and diarrhea, or “stomach flu”) in children, particularly those aged 5-8. The CDC notes that Guillain-Barré syndrome may occur in 1 or 2 people out of every 1 million, but there is not strong evidence that this is actually caused by the flu vaccine. The risk of Guillain-Barré is higher with flu infections.

The MMR (measles – mumps – rubella) vaccine can cause febrile seizures, the IOM found, and can cause an allergic reaction in those with severe allergy. The CDC estimates febrile seizures at 1 in 3,000 doses; I’ve written about the increased risk of febrile seizures when the MMR is delayed.

Children with compromised immune systems are at risk for measles inclusion body encephalitis, an extremely rare condition in immunocompromised individuals that can be fatal. (Immunocompromised individuals should not receive the MMR.) It can also be caused by exposure to the measles virus. The measles vaccine may also cause temporary joint pain, estimated by the CDC to occur to one in four people. There was no evidence for a link between the MMR and autism, confirmed by additional studies since the IOM one. This updated analysis also found moderate evidence for a risk of short-term thrombocytopenic purpura (low platelet levels in the blood just under the skin). The CDC estimates this temporary low platelet count to occur to 1 in 30,000 recipients.

The meningococcal vaccine can cause severe allergic reactions in children who are allergic to ingredients in the vaccine. These are so rare that estimates are unavailable at the CDC, which means they are less than 1 in several million doses. Studies looking for links to fever, muscle pain, headache, severe irritability, severe persistent crying, severe sleepiness or hives found no evidence that any of these were caused by the meningococcal vaccine.

The PCV13 vaccine was not included in the IOM report because this vaccine replaces the previously used PCV7; the new one protects against six additional strains of pneumonia and other pneumococcal disease. A study of more than 200,000 children in the US found an increased risk of febrile seizure with PCV13, occurring with 14 out of every 100,000 doses. When given at the same time as the flu vaccine, the risk of febrile seizure increased to 45 out of 100,000 doses. Remember that febrile seizures, while certainly scary, do not cause brain damage, epilepsy or any other long-term effects.

The rotavirus vaccines, RotaTeq and Rotarix, were not included in the IOM report, but 31 different trials were included in the data of this updated analysis. The serious adverse event most associated with rotavirus vaccines is intussusception, or a telescoping of the bowel where one part of the bowel slides into itself. It can require surgery. Some of the studies in this analysis found no link between intussusception and rotavirus, and others found an increased risk of 1-2 cases out of 100,000 doses for RotaTeq and 5 cases out of 100,000 doses for Rotarix. I have discussed the findings of one of the largest studies on intussusception and rotavirus here.

The varicella (chickenpox) vaccine can cause a breakthrough rash of chickenpox that then leads to pneumonia, meningitis or hepatitis if the vaccine is given to immunocompromised individuals (those with an underlying condition that weakens their immune system – not simply a child whose immune system happens to be fighting another infection). Those with known immunodeficiency conditions should not receive this vaccine.

The chickenpox vaccine can also cause shingles in rare cases, as I mentioned in my recent post about the chickenpox vaccine. This updated analysis found some moderate evidence that immune thrombocytopenic purpura – low platelet count leading to easy bruising – may occur in those aged 11 to 17 who receive this vaccine.

The authors of this review also reported on several studies that looked for – and did not find – an increased risk of leukemia with multiple vaccines given at once.

Also, keep in mind that it is theoretically possible for any vaccine ingredient to cause an extremely rare severe allergic reaction, but such cases have rarely been documented and occur too rarely to find in even huge studies. The CDC estimates these reactions occur in less than 1 per million doses.

Annnnnnnnd that’s all folks. That’s all there’s evidence for. Does that mean this study is the final say on vaccine risks? Absolutely not. Researchers will keep looking, conducting studies to see if there are any side effects that have been overlooked or to find out whether the estimated rates of what we know about changes.

The review was funded by the US Department of Health and Human Services with no pharma funding, and the authors are from the RAND Corporation, the VA Greater Los Angeles Healthcare System, the UCLA David Geffen School of Medicine and Boston’s Children’s Hospital.

Note: For another solid analysis of this study, check out Skeptical Raptor’s post here.