THE INCIDENCE OF NEUTRALIZING ANTIBODIES FOR HUMAN INFLUENZA VIRUS IN THE SERUM OF HUMAN INDIVIDUALS OF DIFFERENT AGES

Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus

The source, timing, and geographical origin of the 1918-1920 pandemic influenza A virus have remained tenaciously obscure for nearly a century, as have the reasons for its unusual severity among young adults. Here, we reconstruct the origins of the pandemic virus and the classic swine influenza and (postpandemic) seasonal H1N1 lineages using a host-specific molecular clock approach that is demonstrably more accurate than previous methods. Our results suggest that the 1918 pandemic virus originated shortly before 1918 when a human H1 virus, which we infer emerged before ∼1907, acquired avian N1 neuraminidase and internal protein genes. We find that the resulting pandemic virus jumped directly to swine but was likely displaced in humans by ∼1922 by a reassortant with an antigenically distinct H1 HA. Hence, although the swine lineage was a direct descendent of the pandemic virus, the post-1918 seasonal H1N1 lineage evidently was not, at least for HA. These findings help resolve several seemingly disparate observations from 20th century influenza epidemiology, seroarcheology, and immunology. The phylogenetic results, combined with these other lines of evidence, suggest that the high mortality in 1918 among adults aged ∼20 to ∼40 y may have been due primarily to their childhood exposure to a doubly heterosubtypic putative H3N8 virus, which we estimate circulated from ∼1889-1900. All other age groups (except immunologically naive infants) were likely partially protected by childhood exposure to N1 and/or H1-related antigens. Similar processes may underlie age-specific mortality differences between seasonal H1N1 vs. H3N2 and human H5N1 vs. H7N9 infections.

Surveillance and Seroepidemiology

Surveillance has been defined as the systematic collection of data pertaining to the occurrence of diseases, the analysis and interpretation of these data, and the dissemination of consolidated and processed information to contributors, programs, and other interested persons. A wide variety of data sources are used for surveillance purposes. Some data sources were designed for the purpose of surveillance while other data sources are used secondarily for surveillance. To improve the timeliness and quality of surveillance data while simultaneously minimizing cost, surveillance networks and Health Information Exchanges (HIEs) were developed. Surveillance networks allow developing countries to participate in surveillance, establishing early warning of outbreaks. HIEs facilitate access to and retrieval of patient clinical data to support more efficient, timely, effective, equitable, and safe healthcare and to enhance case reporting. Seroepidemiology is the systematic collection and testing of blood samples from a target population to identify current and past experiences with infectious diseases by means of biological markers. Data from serological surveys can reveal total burden of current and past, as well as apparent and inapparetnt infections. Surveillance and seroepidemiology have provided critical epidemiologic information to support public health policy at the local, national, and international levels.

Evaluation of mismatched immunity against influenza viruses

Prior immunity against influenza A viruses generates sterilizing immunity against matched (homologous) viruses and varying levels of protection against mismatched (heterologous) viruses of the same or different subtypes. Natural immunity carries the risk of high morbidity and mortality, therefore immunization offers the best preventative measure. Antibody responses against the viral hemagglutinin protein correlate with protection in humans and evidence increasingly supports a role for robust cellular immune responses. By exploiting mismatched immunity, current conventional and experimental vaccine candidates can improve the generation of cross-protective immune responses against heterologous viruses. Experimental vaccines such as virus-like particles, DNA vectors, viral vectors and broadly neutralizing antibodies are able to expand cross-protection through mismatched B- and T-cell responses. However, the generation of mismatched immune responses can also have the opposite effect and impair protective immunity. This review discusses mismatched immunity in the context of natural infection and immunization. Additionally, we discuss strategies to exploit mismatched immunity in order to improve current conventional and experimental influenza A virus vaccines.

THE INCIDENCE OF NEUTRALIZING ANTIBODIES FOR SWINE INFLUENZA VIRUS IN THE SERA OF HUMAN BEINGS OF DIFFERENT AGES

Sera from a very high proportion of the human adults and new-born infants studied neutralized swine influenza virus; sera from children below the age of 12 years seldom exerted such an effect. The results of neutralization experiments with human sera and the virus of swine influenza have been compared with the outcome of similar tests with the virus of human influenza, and it seems evident that the presence of antibodies neutralizing swine influenza virus cannot be deemed the result of repeated exposures to the current human type of virus. From the known history of swine influenza and the similarity of its etiologic virus to that obtained from man it seems likely that the virus of swine influenza is the surviving prototype of the agent primarily responsible for the great human pandemic of 1918, as Laidlaw has already suggested. The presence in human sera of antibodies neutralizing swine influenza virus is believed to indicate a previous immunizing exposure to, or infection with, an influenza virus of the 1918 type.

NEUTRALIZATION TESTS WITH SERA OF CONVALESCENT OR IMMUNIZED ANIMALS AND THE VIRUSES OF SWINE AND HUMAN INFLUENZA

Human and swine influenza viruses were regularly neutralized by their homologous immune sera. However, the sera of animals convalescent from infection with either the swine or human influenza virus possessed little, if any, neutralizing capacity for the heterologous virus. Hyperimmunization of animals against swine influenza virus tended to increase the neutralizing capacity of their sera for human influenza virus, but in an inconstant fashion, whereas repeated inoculations with human influenza virus frequently resulted in sera with strong neutralizing activities against swine influenza virus. These observations serve to emphasize both the immunological distinctiveness and the interrelationships of swine and human influenza viruses.

NEUTRALIZATION OF EPIDEMIC INFLUENZA VIRUS : THE LINEAR RELATIONSHIP BETWEEN THE QUANTITY OF SERUM AND THE QUANTITY OF VIRUS NEUTRALIZED

A linear relationship exists between the logarithm of the quantity of epidemic influenza virus neutralized and the logarithm of the quantity of antiserum which is capable of achieving this result. This relationship is the same for the serum of a ferret convalescent from experimental influenza as for the serum of a rabbit immunized with the virus. By means of the linear relationship between virus and antiserum it is possible to determine a fixed, rather than a relative, value for the neutralizing capacity of a serum.

A serological investigation into the epidemiology of influenza with particular reference to sporadic cases

An investigation has been described in which the complement fixing antibody titres of selected groups of the population have been correlated with the occurrence of epidemic and sporadic influenza and of other cases of respiratory disease which it was thought might be due to the virus of epidemic influenza.

During two non-epidemic seasons the influenza virus could not be isolated from cases of apparent influenza neither could any serological evidence be obtained of the existence of any form of respiratory disease caused by the virus.

With rare exceptions individual antibody titres either remain constant, or if they are high show a tendency to fall during non-epidemic times, and periodic investigations of sample groups each consisting of several hundred sera show a wave-like fluctuation in the titres of the population consisting of a sudden rise followed by a gradual fall, the former being an accompaniment of an epidemic. Such a wave-like fluctuation appears to be an indication of variations in the immunity of the population, and to be responsible in part, at least, for the occurrence of large influenza epidemics.

I wish to acknowledge the help received from all who took part in this investigation, and in particular to thank the students of the University Halls of Residence in Manchester, the numerous doctors who helped by supplying particulars of cases of influenza and to Dr R. W. Fairbrother who has given advice and maintained a constant interest throughout the investigation.

THE ANTIBODY RESPONSE OF HUMAN SUBJECTS VACCINATED WITH THE VIRUS OF HUMAN INFLUENZA

Human influenza virus cultivated in tissue culture medium may be administered subcutaneously or intradermally to human individuals without causing evidence of infection. Subjects so treated develop a good titer of circulating antibodies effective against mouse passage virus and, if antibodies were previously present, vaccination stimulates the production of more antibody. The antibodies so induced persist for at least 5 months, although in this period of time some decline in titer may have begun. The antibody response to vaccination parallels both in extent and persistence that occurring as a result of the naturally acquired disease. The available data do not enable one to evaluate the effect of vaccination in preventing human infection with influenza. It seems not unlikely that the increase in circulating antibody will be accompanied by an increased ability to combat the natural infection.

Resurrected pandemic influenza viruses

Influenza viruses continue to pose a major global public health problem. There is a need to better understand the pathogenicity and transmission of pandemic influenza viruses so that we may develop improved methods for their prevention and control. Reconstruction of the 1918 virus and studies elucidating the exceptional virulence and transmissibility of the virus are providing exciting new insights into this devastating pandemic strain. The primary approach has been to reconstruct and analyze recombinant viruses, in which genes of the 1918 virus are replaced with genes of contemporary influenza viruses of lesser virulence. This review highlights the current status of the field and discusses the molecular determinants of the 1918 pandemic virus that may have contributed to its virulence and spread. Identifying the exact genes responsible for the high virulence of the 1918 virus will be an important step toward understanding virulent influenza strains and will allow the world to better prepare for and respond to future influenza pandemics.

Experimental infection of pigs with the human 1918 pandemic influenza virus

Swine influenza was first recognized as a disease entity during the 1918 "Spanish flu" pandemic. The aim of this work was to determine the virulence of a plasmid-derived human 1918 pandemic H1N1 influenza virus (reconstructed 1918, or 1918/rec, virus) in swine using a plasmid-derived A/swine/Iowa/15/1930 H1N1 virus (1930/rec virus), representing the first isolated influenza virus, as a reference. Four-week-old piglets were inoculated intratracheally with either the 1930/rec or the 1918/rec virus or intranasally with the 1918/rec virus. A transient increase in temperature and mild respiratory signs developed postinoculation in all virus-inoculated groups. In contrast to other mammalian hosts (mice, ferrets, and macaques) where infection with the 1918/rec virus was lethal, the pigs did not develop severe respiratory distress or become moribund. Virus titers in the lower respiratory tract as well as macro- and microscopic lesions at 3 and 5 days postinfection (dpi) were comparable between the 1930/rec and 1918/rec virus-inoculated animals. In contrast to the 1930/rec virus-infected animals, at 7 dpi prominent lung lesions were present in only the 1918/rec virus-infected animals, and all the piglets developed antibodies at 7 dpi. Presented data support the hypothesis that the 1918 pandemic influenza virus was able to infect and replicate in swine, causing a respiratory disease, and that the virus was likely introduced into the pig population during the 1918 pandemic, resulting in the current lineage of the classical H1N1 swine influenza viruses.

Characteristics of a swine recombinant influenza virus isolated in 1980: recombination between swine and the earliest Hong Kong (H3N2) viruses

A recombinant (H1N2, formerly Hsw1N2), A/swine/Ehime/1/80 was found to possess antigenic biological and genomic characteristics different from those of a previous A/swine/Kanagawa/2/78 (H1N2) strain. Five monoclonal antibodies to A/NJ/8/76 differentiated the haemagglutinin molecules of the former virus from the latter, showing that these viruses differed at two-antigenic determinants at least. Immuno-double diffusion tests with antisera to the isolated neuraminidase and neuraminidase-inhibition tests with monoclonal antibodies to different H2N2 and H3N2 viruses revealed that A/swine/Ehime/1/80 strain contained a neuraminidase subunit very similar to that of late human Asian (H2N2) and the earliest Hong Kong (H3N2) viruses. RNA analysis by oligonucleotide mapping suggested that A/swine/Ehime/1/80 may be a recombinant between A/swine/Shizuoka/1/78-like and A/Aichi/2/68 (H3N2)-like viruses. To determine further the gene constellation of this recombinant virus, DNA-RNA hybridizations were performed using DNA segments complementary for swine (H1N1) virus RNA and the entire RNA of three viruses. The molecular hybridization could define the genomic composition of the recombinant, indicating that only the neuraminidase gene of this virus is derived from the earliest Hong Kong (H3N2)-like virus and remaining seven genes are derived from swine (H1N1) virus.

Characterization of a 1980-swine recombinant influenza virus possessing H1 hemagglutinin and N2 neuraminidase similar to that of the earliest Hong Kong (H3N2) virus

A recombinant (H1N2, formerly Hsw 1N2), A/swine/Ehime/1/80 was found to possess antigenic, biological and genomic characteristics different from those of a previous A/swine/Kanagawa/2/78 (H1N2) strain. Five monoclonal antibodies to A/NJ/8/76 definitely differentiated the hemagglutinin molecules of the former virus from the latter, showing that these viruses differed, at least, at two antigenic determinants. Neuraminidase-inhibition tests with monoclonal antibodies to different H2N2 and H3N2 viruses revealed that the A/swine/Ehime/1/80 strain contained a neuraminidase very similar to that of the late human Asian (H2N2) and the earliest Hong Kong (H3N2) viruses. Growth comparison of swine and human isolates indicated that A/swine/Ehime/1/80 and A/swine/Shizuoka/1/78 (H1N1) failed to grow at 42 degrees C, while A/swine/Kanagawa/2/78 and its possible parental virus, A/swine/Kanagawa/4/78 (H1N1) replicated efficiently at this stringent temperature. These results revealed that the viruses having growth characteristics similar to those of avian influenza virus were present in the swine population. RNA analysis by oligonucleotide mapping suggested that A/swine/Ehime/1/80 may be a recombinant between A/swine/Shizuoka/1/78-like and A/Aichi/2/68 (H3N2)-like viruses. To further determine the gene constellation of this recombinant virus, DNA-RNA hybridization was performed by using DNA segments complementary for swine (H1N1) virus RNA and the entire RNAs of three viruses. The molecular hybridization could define the genomic composition of the recombinant, indicating that only the neuraminidase gene of this virus is derived from the earliest Hong Kong (H3N2)-like virus and remaining seven genes from swine (H1N1) virus.

Swine influenza: epizootiological and serological studies

Studies of naturally occurring respiratory diseases in the midwestern parts of the USA showed that swine influenza is still prevalent and that mild forms as well as the classical forms of swine influenza occur. Outbreaks of respiratory disease of unknown etiology that are clinically similar to swine influenza were also found. On some farms, swine influenza occurred first in farrowing pens. It did not occur on some farms where the disease had occurred in previous years. This disappearance may have resulted from the elimination or hyperimmunization of breeder animals or from a change to the raising of swine obtained by caesarean section. Serological studies of swine with natural or experimental infections showed that antibody titres rose gradually for several months. This observation was corroborated in serological studies of sera obtained at the abattoir, which showed that older breeder swine had consistently higher titres than the younger market swine. These results cannot be explained by the lungworm hypothesis proposed by R. E. Shope for the survival and transmission of swine influenzavirus. It is suggested that breeder swine act as convalescent carriers and as the reservoirs of swine influenzavirus between epizootics.

Studies on relationships between human and porcine influenza. 1. Serological evidence of infection in swine in Great Britain with an influenza A virus antigenically like human Hong Kong-68 virus

Serological evidence of infection of swine in Great Britain with an influenza A virus closely related to the human A/Hong Kong/68 (H3N2) variant was detected by a variety of serological tests. The Hong Kong/68 virus was first detected in man in Great Britain in August 1968 and was prevalent in the winters of 1968-69 and 1969-70. There was no evidence that swine had been infected with a Hong Kong/68-like virus before the appearance of the virus in man. The detection of virus-neutralizing antibody and high titres of neuraminidase-inhibiting antibody for Hong Kong/68 virus, and the production of precipitin lines corresponding to influenza A ribonucleoprotein and haemagglutinin and neuraminidase antigens of Hong Kong virus in immunodiffusion tests indicated that the swine sera contained antibody specific for the Hong Kong/68 virus. Evidence suggested that the infection of swine occurred in the early months of 1970. Clinical influenza among swine in Great Britain was not reported during the study period and there was no serological evidence of infection with "classical" swine influenzavirus strains.

Virus of the 1918 influenza pandemic era: new evidence about its antigenic character

In serums of unusually isolated Pacific islanders whose only exposure to influenza occurred during the era of the 1918 pandemic the residual neutralizing antibody was greatest to the PR/8 and BH strains of human type A influenza virus, significantly lower to swine influenza virus, and absent to equine or later human type A virus strains. The pandemic virus was thus antigenically closer to human type A strains isolated during the middle 1930's than to other known influenza virus types.

Antigenic relationship between influenza A viruses of human and animal origin

Reciprocal antigenic relationships between 17 influenza A viruses of human, porcine, equine and avian origin were investigated by haemagglutination-inhibition (HI) and strain-specific complement-fixation (CF). Cross-reactions were observed between the following strains: (a) Equi/1/Prague/1/56, Fowl plague (Dutch strain) and Turkey/England/1/63 (Langham strain); (b) Equi/2/Miami/1/63, Quail/Italy/1117/65, Pheasant/Italy/647/66, Duck/England/1/62 and Turkey/Canada/1/63; (c) A2/Singapore/1/57 and Turkey/Massachussets/65; (d) Swine/S15/30 and Chicken/Scotland/1/59. The results of HI tests performed with post-infection sera showed on the whole narrower specificity than those of HI with hyperimmune sera or those of strain-specific CF. There is clearly no sharp demarcation of antigenic subtypes of influenza A viruses, and studies over a yet wider range of strains are likely to disclose a continuous spectrum of antigenic variation for the whole group. The authors suggest that, in practice, host specificity rather than antigenic specificity may have to be used as the main criterion in classifying influenza A viruses.

Serological epidemiological studies with influenza A viruses

Determinations were made of the age distribution of antibody to swine virus and representatives of the various families of human influenza A virus in 1961–62 collections of human sera and paired sera from forty individuals taken in 1952 and 1963:

(a) The existence of cohorts of the population, each with a dominant antibody type related to strains of virus first encountered in childhood, was confirmed.

(b) The basic epidemiological pattern was similar to that previously detected in 1954. However, it seemed that antibody to swine virus had been reinforced but not antibody to A and A1 strains.

(c) Neutralizing and HI antibodies to A/Equine/Miami/63 virus were detected only in the sera of older people (65 years or over) collected in 1964. No antibodies were found to A/Equine/Prague/56 or two duck viruses.

(d) Relatively constant levels of antibody to A, A1 and A 2 viruses were present in sera from aged persons but antibody to swine virus diminished with age. This could be attributed to a lack of swine antibody in the older females.

Epidemiologic and immunologic significance of age distribution of antibody to antigenic variants of influenza virus

The effects on the antibody content of the population which result from repeated exposure to antigenic variants of influenza viruses have been studied by measuring, with many strains, the antibody content of lots of gamma globulin prepared in different years and the patterns of antibody found in sera collected in 1 year from various age groups. In all samples of gamma globulin collected from 1943 through 1951, high levels of antibody were found with strains of Type A and Type B influenza viruses isolated prior to 1941. The highest levels were found in the more recent collections of gamma globulin. Antibodies to A-prime, and to B strains of 1945 and 1952, were present at low levels in gamma globulin collected prior to the isolation of these viruses. A moderate increase in antibody was observed in the gamma globulin of recent years. The pattern of distribution of antibody by age found with most A-prime strains in serum pools exhibited high levels in infancy and childhood, but after the age of 20, little or no antibody was detected. With Type A strains antibody was usually not observed until the 11th year of age. Thereafter, high levels were present until age 20, when the amount of antibody declines to a moderate and relatively constant level which persists throughout life. Antibody against swine influenza virus did not become detectable until the 29th year. The intermediate antigenic character of a few A-prime isolates was reflected in the antibody pattern obtained with them. Antibody was not found until age 13 with the Lee (1940) strain of Type B influenza virus, but thereafter the level was high. With the type B isolates of 1945 and 1952, antibody became measurable at earlier ages. The present data clearly demonstrate that in the early years of life the range of the antibody spectrum is narrow, and that it becomes progressively broader in later life. A striking correlation was found between what is known of the periods of prevalence of certain strains of influenza viruses and the age of the people in whom strain-specific antibodies are currently found. It has been observed that the age at which antibodies to certain strains are first detectable has progressively advanced with the passage of time. From these data the following immunologic thesis is formulated. The antibody which is acquired during the initial infections of childhood is of limited scope and reflects the dominant antigens of the prevailing strains. The immunity conferred by the initial experiences with influenza is also limited. Successive experiences later in life with viruses of related but differing antigenic make-up result in a composite of antibody which is oriented toward a larger number of the common antigens which comprise influenza virus. These experiences confer a broader immunity which limits infection with, and antibody response to, the more recently encountered strains. The antibody-forming mechanisms appear to be oriented by the initial infections of childhood so that exposures later in life to antigenically related strains result in a progressive reinforcement of the primary antibody. The highest cumulative antibody levels detectable in a particular age group tend, therefore, to reflect the dominant antigens of the virus responsible for the childhood infections of that group. Hence the pattern of antibody distribution determined currently in different age groups provides a serologic recapitulation of past infection with antigenic variants of influenza viruses.