THE PYROGENS OF MYXOVIRUSES. II. RESISTANCE OF INFLUENZA A PYROGENS TO HEAT, ULTRAVIOLET, AND CHEMICAL TREATMENT

Advances in Adjuvanted Influenza Vaccines

The numerous influenza infections that occur every year present a major public health problem. Influenza vaccines are important for the prevention of the disease; however, their effectiveness against infection can be suboptimal. Particularly in the elderly, immune induction can be insufficient, and the vaccine efficacy against infection is usually lower than that in young adults. Vaccine efficacy can be improved by the addition of adjuvants, and an influenza vaccine with an oil-in-water adjuvant MF59, FLUAD, has been recently licensed in the United States and other countries for persons aged 65 years and older. Although the adverse effects of adjuvanted vaccines have been a concern, many adverse effects of currently approved adjuvanted influenza vaccines are mild and acceptable, given the overriding benefits of the vaccine. Since sufficient immunity can be induced with a small amount of vaccine antigen in the presence of an adjuvant, adjuvanted vaccines promote dose sparing and the prompt preparation of vaccines for pandemic influenza. Adjuvants not only enhance the immune response to antigens but can also be effective against antigenically different viruses. In this narrative review, we provide an overview of influenza vaccines, both past and present, before presenting a discussion of adjuvanted influenza vaccines and their future.

Sensitivity of influenza virus to ultraviolet irradiation

BACKGROUND

The measures implemented against the coronavirus pandemic also led to a sharp decline in influenza infections in the 2020/2021 flu season. In the meantime, however, the number of influenza infections has risen again; it is known from history that influenza viruses can also trigger severe pandemics. Therefore, we investigated the efficacy of ultraviolet radiation in the spectral range of 200-400 nm for inactivating influenza viruses.

MATERIALS AND METHODS

The scientific literature was searched for published ultraviolet (UV) irradiation experiments with influenza viruses and the results were standardized by determining the lg-reduction dose. The results were then sorted and analyzed by virus type and wavelength as far as possible.

RESULTS

The scope of the published data sets was limited and revealed large variations with regard to the lg-reduction dose. Only for experiments with influenza viruses in liquid media in the UVC spectral range around 260 nm - the emission range of commonly-used mercury vapor lamps - was there sufficient data to compare virus types. No significant difference between the virus (sub-) types was observed. The lg-reduction dose in this spectral range is 1.75 mJ/cm² (median). It was also shown that influenza viruses are particularly sensitive in the far-UVC spectral range (200-230 nm).

CONCLUSION

UVC, including far-UVC, is suited for influenza virus inactivation as long as the viruses are in UVC-transparent materials. A large difference in the UV sensitivity of different influenza viruses from the last approx. 100 years could not be detected. Thus, it is reasonable to assume that future influenza viruses will also be similarly UV-sensitive or that UV can also inactivate new influenza viruses.

Heat Inactivation of Influenza Viruses—Analysis of Published Data and Estimations for Required Decimal Reduction Times for Different Temperatures and Media

(1) Background: Influenza is a viral infection that has claimed many millions of lives over the past 100 years, and there is always a risk that a new influenza virus will emerge and cause another pandemic. One way to reduce such a potential new influenza virus will be heat inactivation. The question in this study is how much the heat sensitivities of previous influenza viruses differ. If they are very similar, it is expected that a new influenza virus can be inactivated with the same heat parameters as previous influenza viruses. (2) Methods: Through a literature search, published heat inactivation results are compiled and analyzed using Arrhenius models and regression equations for decimal reduction times for different temperatures and media determined. (3) Results: There are about 50 studies on heat inactivation of human and avian influenza viruses so far, showing large differences in heat sensitivity of influenza viruses in different media. However, within a single medium the differences between viruses are rather small. (4) Conclusions: At a temperature of 60 °C, previous influenza viruses can be reduced by 4 or more orders of magnitude within approximately 30 min in almost all media, and this is likely to be true for a potential new influenza virus. Further studies, especially on human influenza viruses, would be desirable.

Interleukin 1 and interleukin 1 inhibitor production by human macrophages exposed to influenza virus or respiratory syncytial virus. Respiratory syncytial virus is a potent inducer of inhibitor activity

Respiratory viral infections are commonly associated with altered immune responses, such as proliferative responses to mitogens and antigens. To examine potential mechanisms, we examined production of IL-1 and IL-1 inhibitors by purified human peripheral blood-derived macrophages exposed to influenza virus or respiratory syncytial virus (RSV). IL-1 and IL-1 inhibitor activities in supernatant fluids from macrophages exposed to the viruses 24 h previously were measured using the standard mouse thymocyte comitogen assay. Crude fluids from macrophages exposed to influenza virus contained substantial IL-1 activity, whereas crude fluids from macrophages exposed to RSV contained marked IL-1 inhibitor activity. Assays with gel filtration-separated fractions revealed that both influenza virus and RSV induced production of both IL-1 and IL-1 inhibitors. Neither IL-2 nor IL-2 inhibitor activities were detected. Thus, effects of human macrophage-derived factors on thymocyte proliferation, or potentially on human lymphocyte proliferation, may reflect the total or net activity of multiple composite factors, the balance of which varies according to the challenge. The data raise the possibility that marked production of IL-1 inhibitor activity in response to RSV plays a role in the clinical recurrence of RSV infection despite the absence of clear evidence for antigenic shift or drift of the virus.

The estimation of host antigen in experimental and commercial influenza subunit preparations by delayed-type hypersensitivity reaction

A delayed-type hypersensitivity (DTH) test has been used to detect the presence of host-specific antigen(s) in highly-purified egg-grown influenza virus and virus components. Cyclophosphamide-treated mice, primed with an extract from allantoic fluid, showed DTH reactions when challenged with whole virus, spikeless virus particles or polymer-type subunits. All commercial egg-grown influenza vaccines showed the presence of host antigen, whereas lipid-free derivatives such as internal virus protein complexes and monomer HA preparations lacked the activity. These findings are discussed in relation to vaccine application.

A surface antigen influenza vaccine. 2. Pyrogenicity and antigenicity

Conventional influenza vaccine containing whole virus particles purified on a zonal centrifuge is pyrogenic and can cause systemic and local adverse side effects. An improved vaccine was therefore prepared which contained only the surface antigens of the virus adsorbed to aluminium hydroxide. The antigenicity of this vaccine was compared with conventional vaccine in chickens. Both vaccines induced similar titres of serum haemagglutination-inhibition and neuraminidase inhibition antibody. The dose response curves, however, were different. The surface antigens at vaccine strength without aluminium hydroxide were of negligible pyrogenicity in rabbits.

Purification of influenza virus with Arcton 113

Arcton 113 has proved effective in the removal of host membrane fragments which cosediment with influenza virus during differential centrifugation. The virus appeared morphologically undamaged by the procedure and retained its infectivity, neuraminidase, and hemagglutinating activity. Pyrogenicity, however, was considerably reduced.

Inhibition of Newcastle disease virus-induced fever in rabbits by cycloheximide

Pretreatment with cycloheximide prevents virus-induced fever in rabbits. Compared to the febrile control animals, CH-pretreated afebrile animals exhibit similar concentrations of endogenous pyrogen in the blood and of prostaglandins of the E-group and adenosine 3', 5'-cycle monophosphate in the cerebrospinal fluid. The fever response seems to involve one or several additional so far unknown mediators which are sensitive to cycloheximide.