Seasonal jab and swine flu

Thursday October 8 2009

A study published recently in the British Medical Journal suggests that the 2008-9 seasonal flu vaccine may offer some level of cross-protection against the swine flu virus. This is an unexpected finding as it was widely assumed that the annual flu jab would offer no protection, particularly against the severe complications of flu.

There are several limitations to this case-control study, which the researchers acknowledge. Among these are the facts that the study was in only a small number of patients, and there are no guarantees that the people given the vaccine were similar enough to the people who did not have it to make the results meaningful.

People in high-risk groups should still receive the swine flu vaccine. Although this study appears to suggest that people who had last winter’s seasonal flu vaccine may have been given some small level of protection, the authors of the research note that this does not mean that the seasonal flu vaccine should replace a specific swine flu vaccine, which they say is “crucial”.

Key points

  • The study was in a small number of patients: 60 confirmed cases of influenza A/H1N1 and 180 uninfected controls from the same respiratory hospital in Mexico.
  • The uninfected patients (control group) were significantly more likely to have had the seasonal flu vaccine. Vaccinated people were also less likely to develop particularly severe forms of swine flu.
  • The chances of having swine flu were lower in people who had received the seasonal flu vaccine (odds ratio 0.27, 95% CI 0.11 to 0.66).
  • The results indicate that the cases and controls may not have been well matched for some important factors, making them less comparable. In particular, controls were more likely than cases to have underlying conditions that could make them more susceptible to have complications from flu infection. The researchers suggested that this was because the controls were patients at a hospital specialising in respiratory diseases.
  • None of the eight patients with swine flu who had received the seasonal flu vaccine died. Whereas 18 of the 52 unvaccinated people (35%) with swine flu died. It is possible that the people who had contracted swine flu who had chosen to have the seasonal flu vaccine were generally healthier people than those who did not.
  • The case-control design and small size of the study makes it prone to uncertainties.
  • It is not certain why the seasonal flu vaccine might give some protection. The researchers say their data supports the theory that individuals may have been exposed to a flu virus or immunisation that produced antibodies that have some effect against the swine flu virus, and that the seasonal vaccination boosted this immunity.

Where was the article published?

This research was carried out by Professor Lourdes García-García and colleagues from the Instituto Nacional de Salud Pública, Cuernavaca and elsewhere in Mexico. The study was published in the BMJ and supported by the Mexican Ministry of Health.

What kind of study was this?

This case-control study investigated whether there was an association between the 2008-9 seasonal flu vaccine (trivalent inactivated vaccine) and cases of influenza A/H1N1 from March to May 2009 in Mexico.

Sixty patients were treated for (laboratory-confirmed) influenza A/H1N1 in the same specialist respiratory hospital in Mexico. These cases were matched to 180 controls with other diseases living in Mexico City or the State of Mexico and who were also treated at the hospital. Steps were taken to ensure that the controls had the same proportion of people of different ages and socioeconomic groups (frequency matching) and that the controls were not suffering from influenza-like illness or pneumonia.

The cases had been clinically diagnosed with influenza and had a respiratory tract sample that tested positive for novel influenza A/H1N1. This was taken using the accepted test in use at the time. Most of the controls were in hospital for ear, nose or throat surgery, asthma or testing for obstructive sleep apnoea.

Both cases and controls were asked if they had received the flu vaccine for the 2008-9 winter season. Interviewers (who were aware of whether the patient had had swine flu or not) used a standardised set of questions, either through a face-to-face interview or over the phone with either the patient or their close relatives.

The outcome measured was the number of cases of influenza A/H1N1. The researchers examined several features in the backgrounds of the cases and controls that increased or reduced the risk of infection with influenza A/H1N1. This information was obtained from patient files after diagnosis had been made and included data on age, sex, socioeconomic status, medical conditions, admission to hospital, use of invasive mechanical ventilation and clinical outcome.

Socioeconomic status was taken from the patients’ clinical charts, which recorded the social workers’ evaluation. This was calculated on the basis of annual income and the formal education of each household member, number of people in each household and characteristics of the household. The data was then analysed using the accepted method of regression analysis, a modelling technique.

What are the research findings?

Many of the 60 people with swine flu (63%) were between 21 and 60 years of age. The controls were age matched to the cases.

Controls were more likely than cases to have chronic conditions that conferred a higher risk of
Influenza-related complications. This may have been because they were selected from a respiratory hospital population that had a high incidence of these long-term conditions.

In the model, the risk of having influenza A/H1N1 was significantly reduced in people who received the 2008-9 seasonal trivalent inactivated vaccine (odds ratio 0.27, 95% CI 0.11 to 0.66). Vaccine effectiveness was 73% (95% confidence interval 34% to 89%). This means that the vaccine was linked to a reduction of 73% in the odds of developing swine flu.

The model also showed that the chances of having swine flu were also lower for people with underlying conditions that could make them more susceptible to have complications from flu infection, independent of their vaccination status (odds ratio 0.15, 95% CI 0.08 to 0.30).

None of the eight patients with swine flu who had received the seasonal flu vaccine died. Whereas 18 of the 52 unvaccinated people (35%) with swine flu died.

What is the implication and importance of this?

Dr Menno de Jong from the Academic Medical Centre of the University of Amsterdam discusses this study in an editorial published in the same journal. He agrees with the authors that the results “do not mean that there is no need for a specific vaccine against swine flu”. This is an important point at a time when the UK is preparing for a nation-wide vaccination campaign.

Another recent study has shown that, in blood tests at least, there was some evidence of immune response against the new H1N1 virus after seasonal vaccination in some adults. As such, this is not the only research to suggest that the seasonal flu vaccine gives some protection against swine flu.

Another point of note is that the study was conducted in Mexico in the early days of the epidemic; results may be different in other countries and the level of protection offered may change as the pandemic has progressed.

In addition, the benefit was mostly seen in the 41 to 60-year-olds (63%). This is an unusual age group as vaccinations are often considered to have the greatest benefits in more elderly groups (aged over 60). Only five cases and 15 controls were over 60. The results may not apply to older or younger age groups.

More research will be needed to evaluate the degree of protection offered by the seasonal flu vaccine. Expert advice is that if you are in a high-risk group you should get the swine flu vaccine as the best prevention. This study provides limited evidence and a small chance that a seasonal flu jab might protect you against the complications of H1N1 swine flu.

Analysis by Bazian
Edited by NHS Choices