A suite of three influenza vaccine reviews has been updated:
- Vaccines for preventing influenza in healthy adults.
- Vaccines for preventing influenza in healthy children.
- Vaccines for preventing influenza in the elderly.
Vaccines for preventing influenza in healthy adults
Plain language summary
Review aim
The aim of this Cochrane Review, first published in 1999, was to summarise research that looks at the effects of immunising healthy adults with influenza vaccines during influenza seasons. We used information from randomised trials comparing vaccines with dummy vaccines or nothing. We focused on the results of studies looking at vaccines based on inactivated influenza viruses, which are developed by killing the influenza virus with a chemical and are given by injection through the skin. We evaluated the effects of vaccines on reducing the number of adults with confirmed influenza and the number of adults who had influenza-like symptoms such as headache, high temperature, cough, and muscle pain (influenza-like illness, or ILI). We also evaluated hospital admission and harms arising from the vaccines. Observational data included in previous versions of the review have been retained for historical reasons but have not been updated due to their lack of influence on the review conclusions.
What was studied in this review?
Over 200 viruses cause ILI, which produces the same symptoms (fever, headache, aches, pains, cough, and runny nose) as influenza. Without laboratory tests, doctors cannot distinguish between ILI and influenza because both last for days and rarely cause serious illness or death. The types of virus contained in influenza vaccines are usually those that are expected to circulate in the following influenza seasons, according to recommendations of the World Health Organization (seasonal vaccine). Pandemic vaccine contains only the virus strain that is responsible of the pandemic (i.e. the type A H1N1 for the 2009 to 2010 pandemic).
Main results
We found 52 clinical trials of over 80,000 adults. We were unable to determine the impact of bias on about 70% of the included studies due to insufficient reporting of details. Around 15% of the included studies were well designed and conducted. We focused on reporting of results from 25 studies that looked at inactivated vaccines. Injected influenza vaccines probably have a small protective effect against influenza and ILI (moderate-certainty evidence), as 71 people would need to be vaccinated to avoid one influenza case, and 29 would need to be vaccinated to avoid one case of ILI. Vaccination may have little or no appreciable effect on hospitalisations (low-certainty evidence) or number of working days lost.
We were uncertain of the protection provided to pregnant women against ILI and influenza by the inactivated influenza vaccine, or this was at least very limited.
The administration of seasonal vaccines during pregnancy showed no significant effect on abortion or neonatal death, but the evidence set was observational.
Key messages
Inactivated vaccines can reduce the proportion of healthy adults (including pregnant women) who have influenza and ILI, but their impact is modest. We are uncertain about the effects of inactivated vaccines on working days lost or serious complications of influenza during influenza season.
How up to date is this review?
The evidence is current to 31 December 2016.
Vaccines for preventing influenza in healthy childrenPlain language summary
Review aim
The aim of this Cochrane Review, first published in 2007, was to summarise research on immunising healthy children up to the age of 16 with influenza vaccines during influenza seasons. We used randomised trials comparing either one of two types of vaccines with dummy vaccines or nothing. One type of vaccine is based on live but weakened influenza viruses (live attenuated influenza vaccines) and is given via the nose. The other is prepared by killing the influenza viruses with a chemical (inactivated virus) and is given by injection through the skin. We analysed the number of children with confirmed influenza and those who had influenza-like illness (ILI) (headache, high temperature, cough, and muscle pain) and harms from vaccination. Future updates of this review will be made only when new trials or vaccines become available. Data from 33 observational studies included in previous versions of the review have been retained for historical reasons but have not been updated due to their lack of influence on the review conclusions.
Key messages
Live attenuated and inactivated vaccines can reduce the proportion of children who have influenza and ILI. Variation in the results of studies means that we are uncertain about the effects of these vaccines across different seasons.
What was studied in this review?
Over 200 viruses cause ILI and produce the same symptoms (fever, headache, aches, pains, cough, and runny nose) as influenza. Doctors cannot distinguish between them without laboratory tests because both last for days and rarely cause serious illness or death.
The types of virus contained in the vaccines are usually those that are expected to circulate in the following influenza seasons, according to recommendations of the World Health Organization (seasonal vaccine). Pandemic vaccine contains only the virus strain that is responsible for the pandemic (e.g. the type A H1N1 for the 2009 to 2010 pandemic).
Main results
We found 41 randomised studies. Most studies included children older than two years of age and were conducted in the USA, Western Europe, Russia, and Bangladesh.
Compared with placebo or do nothing, live attenuated vaccines probably reduced the proportion of children who had confirmed influenza from 18% to 4% (moderate-certainty evidence), and may reduce ILI from 17% to 12% (low-certainty evidence). Seven children would need to be vaccinated for one child to avoid influenza, and 20 children would need to prevent one child from experiencing an ILI. We found data from one study that showed similar risk of ear infection in the two groups. There was insufficient information available to assess school absence and parents needing to take time off work. We found no data on hospitalisation, and harms were not consistently reported.
Compared with placebo or no vaccination, inactivated vaccines reduce the risk of influenza from 30% to 11% (high-certainty evidence), and they probably reduce ILI from 28% to 20% (moderate-certainty evidence). Five children would need to be vaccinated for one child to avoid influenza, and 12 children would need to be vaccinated to prevent one case of ILI. The risk of otitis media is probably similar between vaccinated children and unvaccinated children (31% versus 27%, moderate-certainty evidence). There was insufficient information available to assess school absenteeism due to very low-certainty evidence from one study. We identified no data on parental working time lost, hospitalisation, fever, or nausea.
One brand of monovalent pandemic vaccine was associated with a sudden loss of muscle tone triggered by the experience of an intense emotion (cataplexy) and a sleep disorder (narcolepsy) in children.
Only a few studies were well designed and conducted, and the impact of studies at high risk of bias varied across the outcomes evaluated. Influenza and otitis media were the only outcomes where our confidence in the results was not affected by bias.
How up to date is this review?
The evidence is current to 31 December 2016.
Vaccines for preventing influenza in the elderlyPlain language summary
Review aim
The aim of this Cochrane Review, first published in 2006, was to summarise research that looks at the effects of immunising the elderly (those aged 65 years or older) with influenza vaccine during influenza seasons. We used information from randomised trials comparing influenza vaccine with dummy vaccine or with nothing. The influenza vaccines were prepared by treating influenza viruses with a chemical that kills the virus (inactivated virus), and the vaccination was given by injection through the skin. We were interested in showing the effects of vaccines on reducing the number of elderly with confirmed influenza, the number who had influenza-like symptoms such as headache, high temperature, cough, and muscle pain (influenza-like illness, of ILI), and harms from vaccination. We looked for evidence of the impact of influenza or ILI such as hospital admission, complications, and death. We will update this review in the future only when new trials or vaccines become available.
Observational data from 67 studies included in previous versions of the review have been retained for historical reasons but have not been updated because of their lack of influence on the review conclusions.
What was studied in this review?
Over 200 viruses cause ILI, producing the same symptoms (fever, headache, aches, pains, cough, and runny nose). Without laboratory tests, doctors cannot distinguish between viruses, as they last for days and rarely lead to serious illness. At best, vaccines are only effective against influenza A and B, which represent about 5% of all circulating viruses. Inactivated vaccine is prepared by treating influenza viruses with a specific chemical agent that 'kills' the virus. Final preparations may contain either the complete viruses (whole-virion vaccine) or the active part of them (split or subunit vaccines). These vaccines are typically administered by injection through the skin. The virus strains contained in the vaccine are usually those that are expected to circulate in the following epidemic seasons (two type A and one or two B strains), which are recommended by the World Health Organization (seasonal vaccine). Pandemic vaccine contains only the virus strain that is responsible for the pandemic (e.g. the type A H1N1 for the 2009 to 2010 pandemic).
Key messages
Inactivated vaccines can reduce the proportion of elderly who have influenza and ILI. Data on deaths were sparse, and we found no data on hospitalisations due to complications. However, variation in the results of studies means we cannot be certain about how big a difference these vaccines will make across different seasons.
Main results
We found eight randomised controlled trials (over 5000 people), of which four assessed harms. The studies were conducted in community and residential care settings in Europe and the USA between 1965 and 2000.
Older adults receiving the influenza vaccine may experience less influenza over a single season, from 6% to 2.4%, meaning that 30 people would need to be vaccinated with inactivated influenza vaccines to avoid one case of influenza. Older adults also probably experience less ILI, from 6% to 3.5%, meaning that 42 people would need to be vaccinated to prevent one case of ILI. The amount of information on pneumonia and mortality was limited. Data were insufficient to be certain about the effect of vaccines on mortality. No cases of pneumonia occurred in one study that reported this outcome, and no data on hospitalisations were reported. We do not have enough information to assess harms relating to fever and nausea in this population.
The impact of influenza vaccines in older people is modest, irrespective of setting, outcome, population, and study design.
How up to date is this review?
The evidence is current to 31 December 2016.