Assessment of the efficacy of commercially available and candidate vaccines against a pandemic H1N1 2009 virus

Abstract

Background: The emergence and global spread of the pandemic H1N1 2009 influenza virus have raised questions regarding the protective effect of available seasonal vaccines and the efficacy of a newly produced matched vaccine.

Methods: Ferrets were immunized with the 2008-2009 formulations of commercially available live attenuated (FluMist; MedImmune) or split-inactivated (Fluviral; GlaxoSmithKline) vaccines, a commercial swine vaccine (FluSure; Pfizer), or a laboratory-produced matched inactivated whole-virus vaccine (A/Mexico/InDRE4487/2009). Adaptive immune responses were monitored, and the animals were challenged with A/Mexico/InDRE4487/2009 after 5 weeks.

Results: Only animals that received the swine or matched vaccines developed detectable hemagglutination-inhibiting antibodies against the challenge virus, whereas a T cell response was exclusively detected in animals vaccinated with FluMist. After challenge, all animals had high levels of virus replication in the upper respiratory tract. However, preexisting anti-pandemic H1N1 2009 antibodies resulted in reduced clinical signs and improved survival. Surprisingly, FluMist was associated with a slight increase in mortality and greater lung damage, which correlated with early up-regulation of interleukin-10.

Conclusions: The present study demonstrates that a single dose of matched inactivated vaccine confers partial protection against a pandemic H1N1 2009 virus, and it suggests that a higher dose or prime-boost regimen may be required. The consequences of mismatched immunity to influenza merit further investigation.

Figure 1.

Humoral and cellular immune responses elicited by the different vaccines. Quantification of hemagglutination-inhibiting antibody titers against H1N1 A/Mexico/InDRE4487/2009 (MX10) (A) and H1N1 A/ Brisbane/59/07 (B) over the first 21 days after immunization. Data points denote the mean of all values obtained for the respective groups, and error bars denote the standard error. C, Proliferation activity of peripheral blood mononuclear cells (PBMCs) isolated on day 10 after immunization, upon stimulation with 3 pools of overlapping peptides covering the nucleocapsid (NP), neuraminidase (NA), and hemagglutinin (HA) proteins of the H1N1 strain Brevig Mission/1/1918. The proliferation index denotes the ratio of influenza peptide pool-stimulated and Ebola control peptide- stimulated values. The average proliferation observed in each group for a respective pool is shown, and the individual pools are denoted by black, gray, or white bars. Ctl, nonimmunized control group; HAI, hemagglutination-inhibiting antibodies.

Figure 2.

Clinical assessment after challenge with MX10. Five weeks after immunization, the animals were challenged with 10⁵ 50% tissue culture infectious doses of MX10. Temperature (A) and weight (B) were determined daily, and clinical signs (C) were scored at least every third day over the course of the disease. Data points denote the mean of all values obtained for the respective groups, and error bars denote the standard error. D, Survival curve of animals in the different groups. Ctl, nonimmunized control group.

Figure 3.

Gross pathological changes in the lungs. On day 9 after infection, the lungs shown were collected from animals in the FluMist (MedImmune), pH1N1inact, Fluviral (GlaxoSmithKline), and control groups that had reached experimental end points and from an animal randomly selected from the FluSure (Pfizer) group.

Figure 4.

Nasal wash titers and viral load. Nasal wash titers (A) and the viral copy numbers in nasal wash (B) were quantified on days 1, 3, 6, 9, and 16 after challenge. Data points denote the mean of all values obtained for the respective groups, and error bars denote the standard error.

Figure 5.

Relative quantification of cytokine messenger RNA (mRNA) induction. Changes in cytokine mRNA levels were determined by semiquantitative real-time reverse-transcription polymerase chain reaction in nasal wash RNA or RNA isolated from lung tissue harvested on day 9. Ten nanograms of RNA were used for each reaction, and the fold change was calculated using the comparative cycle threshold (ΔΔCt) method. Columns denote the mean of all values obtained for the respective group, and error bars denote the standard error. Ctl, nonimmunized control group; dpi, days post infection; IFN, interferon; IL, interleukin.