The vaccination coverage required to establish herd immunity against influenza viruses

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Abstract

Objective

1) To determine the influenza vaccination coverage required to establish herd immunity, and 2) to assess whether the percentages of vaccination coverage proposed and those registered in the United States and Europe are sufficient to establish herd immunity.

Methods

The vaccination coverage required to establish herd immunity was determined by taking into account the number of secondary cases per infected case (Ro) and the vaccine effectiveness.

Results

The required percentage that would have been required to establish herd immunity against previous influenza viruses ranged from 13% to 100% for the 1918–19, 1957–58, 1968–69 and 2009–10 pandemic viruses, and from 30% to 40% for the 2008–09 epidemic virus. The objectives of vaccination coverage proposed in the United States — 80% in healthy persons and 90% in high-risk persons — are sufficient to establish herd immunity, while those proposed in Europe — only 75% in elderly and high-risk persons — are not sufficient. The percentages of vaccination coverage registered in the United States and Europe are not sufficient to establish herd immunity.

Conclusion

The influenza vaccination coverage must be increased in the United States and Europe in order to establish herd immunity. It is necessary to develop new influenza prevention messages based on herd immunity.

Highlights

► Vaccination coverage required to establish herd immunity for previous and new influenza viruses. ► Proposed vaccination coverage objectives are sufficient to establish herd immunity. ► Percentages of vaccination coverage registered are not sufficient to establish herd immunity. ► Influenza vaccination coverage must be increased. ► It is important to vaccinate low-risk persons to protect persons with weak immune systems.

Introduction

The epidemiology of influenza has three characteristics: 1) influenza epidemics occur every year during winter months, 2) potentiality for causing pandemics, 3) rates of serious illness are highest among persons aged > 65 years, children aged < 2 years, and persons with high-risk conditions, and 4) health and economic impact of epidemics and pandemics depend on the genetic variability of influenza viruses and the percentage of susceptible persons in the population.

Influenza vaccination is the primary method for the prevention and control of influenza, with antiviral drugs as an adjunct to vaccination. The Advisory Committee on immunization Practices (ACIP) of the Center for Disease Control of the United States (CDC, 2010) recommends influenza vaccination for persons older than 6 months, but specifically singles out the following target groups: persons aged ≥ 50 years, persons aged < 50 years at increased risk for influenza-related complications (including children aged 6–23 months and pregnant women), health-care workers, and household contacts of persons at high-risk (including children aged 0–23 months). First, influenza vaccination can reduce the risk for influenza and influenza-related complications, physician visits, hospitalization and death in both healthy and high-risk persons (Centers for Disease Control, Prevention (CDC), 2010, Plans-Rubió, 2007). Second, influenza vaccination can block the transmission of influenza viruses in the community by establishing herd immunity (Anderson, 1992, Centers for Disease Control, Prevention (CDC), 2010).

The annual influenza vaccination coverage objectives in the United States are 80% for healthy persons and 90% for high-risk persons and health professionals (US Department of Health and Human Services, 2011). In Europe, the vaccination coverage objective is 75% for elderly and high-risk persons (WHO, 2003). Influenza vaccination levels increased during the 1990s, but they were lower than 50% in healthy and high-risk persons in the United States and Europe (Blank and Szucs, 2009, Centers for Disease Control, Prevention (CDC), 2010, Maurer and Harris, 2011, Mereckiene et al., 2008a, Mereckiene et al., 2008b, Morabia and Costanza, 2010).

It is not known whether the proposed and achieved vaccination coverage can establish the herd immunity necessary to block transmission of influenza viruses in the population. A recent study showed that an inactivated influenza vaccine could reduce community outbreaks by establishing herd immunity (Loeb et al., 2010). Although the study did not assess the relationship between vaccination coverage and herd immunity, it offered rigorous proof of the ability of the inactivated influenza vaccine to induce herd immunity.

The objectives of this study were: 1) to assess the relationship between influenza vaccination coverage and herd immunity, 2) to determine the vaccination coverage required to establish herd immunity for previous and future influenza viruses, and 3) to assess whether the percentages of influenza vaccination coverage proposed and those registered in the United States and Europe are sufficient to establish herd immunity against epidemic viruses.

Section snippets

The herd immunity threshold

The herd immunity needed to interrupt transmission of influenza viruses in a population is established when the prevalence of protected persons (I) is higher than the “herd immunity threshold” (I > Ic) (Anderson, 1992, Anderson and May, 1995, Plans-Rubió, 2010). When this occurs, transmission of influenza viruses in blocked in the community, but when the prevalence is lower than the threshold, the number of infections grows exponentially, spreading the disease in the community.

The herd immunity

Vaccination coverage required to establish herd immunity for previous influenza viruses

The herd immunity thresholds in terms of prevalence of protected persons required to establish herd immunity (Ic) ranged from 33% to 73% for the first pandemic viruses and from 9% to 29% for the 2009–10 pandemic and the 2008–09 epidemic viruses (Table 1).

The vaccination coverage that would have been required to establish herd immunity for previous influenza viruses ranged from 0% to 100%, depending on vaccine effectiveness, setting and the prevalence of persons already protected in the

Discussion

This study has assessed the vaccination coverage that could be required to establish herd immunity against future influenza viruses, and the vaccination coverage required for previous influenza viruses. The study suggests that a higher levels of vaccination coverage could be required for influenza viruses with Ro  2, if the effectiveness of the available vaccine is > 60%, while a 50% coverage could be sufficient for influenza viruses with Ro  1.5 with a vaccine effectiveness > 60%.

The study

Conclusions

According to the model, the influenza vaccination coverage objectives proposed in the United States are sufficient to establish herd immunity against most influenza viruses, while those proposed in Europe would not be sufficient. The influenza vaccination coverage must be increased in the United States and Europe in order to establish herd immunity. It is necessary to develop new influenza prevention messages based on herd immunity.

Conflict of interest statement

The authors declare that there is no conflict of interest.

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