Question: When giving live vaccines, who is most endangered by them?
About the article "Virus", section "Vaccines".

Candidate passages:
Vaccination is a cheap and effective way of preventing infections by viruses.
Vaccines were used to prevent viral infections long before the discovery of the actual viruses.
Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps and rubella.
Smallpox infections have been eradicated.
Vaccines are available to prevent over thirteen viral infections of humans, and more are used to prevent viral infections of animals.
Vaccines can consist of live-attenuated or killed viruses, or viral proteins (antigens).
Live vaccines contain weakened forms of the virus, which do not cause the disease but, nonetheless, confer immunity.
Such viruses are called attenuated.
Live vaccines can be dangerous when given to people with a weak immunity, (who are described as immunocompromised), because in these people, the weakened virus can cause the original disease.
Biotechnology and genetic engineering techniques are used to produce subunit vaccines.
These vaccines use only the capsid proteins of the virus.
Hepatitis B vaccine is an example of this type of vaccine.
Subunit vaccines are safe for immunocompromised patients because they cannot cause the disease.
The yellow fever virus vaccine, a live-attenuated strain called 17D, is probably the safest and most effective vaccine ever generated.
Vaccines typically contain one or more adjuvants, used to boost the immune response.
Tetanus toxoid, for instance, is usually adsorbed onto alum.
This presents the antigen in such a way as to produce a greater action than the simple aqueous tetanus toxoid.
People who get an excessive reaction to adsorbed tetanus toxoid may be given the simple vaccine when time for a booster occurs.
In the preparation for the 1990 Gulf campaign, Pertussis vaccine (not acellular) was used as an adjuvant for Anthrax vaccine.
This produces a more rapid immune response than giving only the Anthrax, which is of some benefit if exposure might be imminent.
Vaccines may also contain preservatives to prevent contamination with bacteria or fungi.
Until recent years, the preservative thimerosal was used in many vaccines that did not contain live virus.
As of 2005, the only childhood vaccine in the U.S. that contains thimerosal in greater than trace amounts is the influenza vaccine,which is currently recommended only for children with certain risk factors.
Single-dose Influenza vaccines supplied in the UK do not list Thiomersal (its UK name) in the ingredients.
Preservatives may be used at various stages of production of vaccines, and the most sophisticated methods of measurement might detect traces of them in the finished product, as they may in the environment and population as a whole. 
Vaccine overload is the notion that giving many vaccines at once may overwhelm or weaken a child's immature immune system and lead to adverse effects.
Although the scientific evidence strongly contradicts this idea, some parents of autistic children believe that vaccine overload causes autism.
The resulting controversy has caused many parents to delay or avoid immunizing their children.
Such parental misperceptions are major obstacles towards immunization of children.
The concept of vaccine overload is flawed on several levels.
Despite the increase in the number of vaccines over recent decades, improvements in vaccine design have reduced the immunologic load from vaccines; the total number of immunological components in the 14 vaccines administered to US children in 2009 is less than 10% of what it was in the 7 vaccines given in 1980.
A study published on 2013 found no correlation between autism and the antigen number in the vaccines the children were administered up to the age of two.
Of the 1008 children in this study, a quarter of them were diagnosed with autism were born between 1994 and 1999, when the routine vaccine schedule could contain more than 3000 antigens (in a single shot of DTP vaccine).
The vaccine schedule in 2012 contains several more vaccines but the number of antigens the child is exposed to by the age of two is 315.
Vaccines pose a minuscule immunologic load compared to the pathogens naturally encountered by a child in a typical year; common childhood conditions such as fevers and middle-ear infections pose a much greater challenge to the immune system than vaccines, and studies have shown that vaccinations, and even multiple concurrent vaccinations, do not weaken the immune system or compromise overall immunity.
The lack of evidence supporting the vaccine overload hypothesis, combined with these findings directly contradicting it, have led to the conclusion that currently recommended vaccine programs do not "overload" or weaken the immune system.
Any experiment based on withholding vaccines from children has been considered unethical, and observational studies would likely be confounded by differences in health-care-seeking behaviours of under-vaccinated children.
Thus, no study directly comparing rates of autism in vaccinated vs. un-vaccinated children has been done.
However, the concept of vaccine overload is biologically implausible, vaccinated and unvaccinated children have the same immune response to non-vaccine related infections, and autism is not an immune-mediated disease, so claims that vaccines could cause it by overloading the immune system goes against current knowledge of the pathogenesis of autism.
As such, the idea that vaccines cause autism has been effectively dismissed by the weight of current evidence.
A 2011 journal article described the vaccine-autism connection as "the most damaging medical hoax of the last 100 years". 
Prevention and treatment of viral disease in humans and other animals.
Vaccination is a way of preventing diseases caused by viruses.
Vaccines simulate a natural infection and its associated immune response, but do not cause the disease.
Their use has resulted in a dramatic decline in illness and death caused by infections such as polio, measles, mumps and rubella.
Vaccines are available to prevent over thirteen viral infections of humans and more are used to prevent viral infections of animals.
Vaccines may consist of either live or killed viruses.
Live vaccines contain weakened forms of the virus, but these vaccines can be dangerous when given to people with weak immunity.
In these people, the weakened virus can cause the original disease.
Biotechnology and genetic engineering techniques are used to produce "designer" vaccines that only have the capsid proteins of the virus.
Hepatitis B vaccine is an example of this type of vaccine.
These vaccines are safer because they can never cause the disease. 
A vaccine is assessed by its "efficacy"; the extent to which it reduces risk of disease under controlled conditions, and its "effectiveness", the observed reduction in risk after the vaccine is put into use.
In the case of influenza, effectiveness is expected to be lower than the efficacy because it is measured using the rates of influenza-like illness, which is not always caused by influenza.
Influenza vaccines generally show high efficacy, as measured by the antibody production induced in animal models or vaccinated people, or most rigorously, by immunizing healthy adult volunteers and then challenging them with virulent influenza virus.
However, studies on the effectiveness of flu vaccines in the real world are uniquely difficult; vaccines may be imperfectly matched, virus prevalence varies widely between years, and influenza is often confused with other influenza-like illnesses.
However, in most years (16 of the 19 years before 2007), the flu vaccine strains have been a good match for the circulating strains, and even a mis-matched vaccine can often provide cross-protection.
Nevertheless, multiple clinical trials of both live and inactivated influenza vaccines against seasonal influenza have been performed and their results pooled and analyzed in several 2012 meta-analyses.
Studies on live vaccines have very limited data, but these preparations may be more effective than inactivated vaccines.
The meta-analyses examined the efficacy and effectiveness of inactivated vaccines against seasonal influenza in adults, children, and the elderly.
In adults, vaccines show a three-quarters reduction in risk of contracting influenza (4% influenza rate among the unvaccinated versus 1% among vaccinated persons) when the vaccine is perfectly matched to the virus and a one-half reduction (2% get flu without vaccine versus 1% with vaccine) when it is not, but no significant effect on the rate of hospitalization.
However, the risk of serious complications from influenza is small in adults, so unless the effect from vaccination is large it might not have been detected.
In children, vaccines again showed high efficacy, but low effectiveness in preventing "flu-like illness".
In children under the age of two the data are extremely limited, but vaccination appeared to confer no measurable benefit.
In the elderly, while many individual studies show effectiveness, the overall evidence is still insufficient evidence to draw clear conclusions on the effectiveness of vaccination, including a new high-dose flu vaccine specifically formulated to provide a larger immune response.
Available evidence indicates that the high-dose vaccine produces a stronger immune response, and a study designed to determine the effectiveness of Fluzone High-Dose in preventing illness from influenza compared with Fluzone is expected to be completed in 2014–2015.
During an influenza pandemic, where a single strain of virus is responsible for illnesses, an effective vaccine could produce a large decrease in the number of cases and be highly effective in controlling an epidemic.
However, such a vaccine would have to be produced and distributed rapidly to have maximum effect.
Such distribution challenges may be met with good success.
Overall, vaccines against the 2009 H1N1 influenza pandemic were found to be effective in a Scottish study.
A 2011 meta-study published in the journal "The Lancet", "Efficacy and Effectiveness of Influenza Vaccines," analyzed 31 prior studies on the effectiveness of influenza vaccination trials conducted between 1967 and 2011.
The analysis found that flu shots were efficacious 67 percent of the time; the populations that benefited the most were HIV-positive adults ages 18 to 55 (76 percent), healthy adults ages 18 to 46 (approximately 70 percent) and healthy children ages 6 to 24 months (66 percent).
The group most vulnerable to non-pandemic flu, the elderly, is also the least to benefit from the vaccine.
There are multiple reasons behind this steep decline in vaccine efficacy, the most common of which are the declining immunological function and frailty associated with advanced age.
In a non-pandemic year, a person in the United States aged 50–64 is nearly ten times more likely to die an influenza-associated death than a younger person, and a person over age 65 is over ten times more likely to die an influenza-associated death than the 50–64 age group.
As mortality is also high among infants who contract influenza, the household contacts and caregivers of infants should be vaccinated to reduce the risk of passing an influenza infection to the infant.
Data from the years when Japan required annual flu vaccinations for school-aged children indicate that vaccinating children—the group most likely to catch and spread the disease—has a strikingly positive effect on reducing mortality among older people, due to herd immunity: one life saved for every 420 children who received the flu vaccine.
However, a 2010 Cochrane review found that the same benefit did not extend to vaccinating health care workers working with elderly patients in long-term care facilities.
In working adults, by contrast, Cochrane found that vaccination reduced both influenza symptoms and working days lost, without affecting transmission or influenza-related complications.