immunity to influenza virus infection induced by heterologous, inactivated vaccines

Med. Microbiol. Immunol. 166, 99--108 (1978) �9 "~-~ f-~'~ ~ r dlcal : !

Microbiology and i Lmmun0!ogy LI ioj �9 by Springer-Verlag 1978

Immunity to Influenza Virus Infection Induced by Heterologous, Inactivated Vaccines

C.W. Potter 1, R. Jennings 1, and K. Nicholson 2

1 Department of Virology, University of Sheffield Medical School, Beech Hill Road, Sheffield $10 2RX, England

2 Division of Communicable Diseases, Clinical Research Centre, Watford Road, Harrow, Middlesex HA1 3U J, England

Abstract. The ability of inactivated influenza A vaccines to induce serum HI antibody and immunity to challenge infection was studied in hamsters and in volunteers. Groups of hamsters were immunized with 200 IU of influenza virus A/Scotland/74, A/Port Chalmers/73, A/England/72, or A/Hong Kong/68. The serum HI antibody response of animals to, and immunity to challenge infection was directly related to the known relationship between the vaccine and test viruses. Thus, hamsters given A/Hong Kong/68 or A/England/72 vaccine produced serum HI antibody and immunity to A/Hong Kong virus infection, and animals given A/Scotland/74, A/Port Chalmers/73, and A/England/72 produced antibody and immunity to A/Scotland infection.

In a volunteer study, groups of students were immunized with 400 IU of the same vaccines as used above. The ability to infect these volunteers with WRL 105 virus given 4 weeks later was directly related to the vacci.ne-induced serum HI antibody to the challenge virus. The highest titers of serum HI antibody to A/Scotland virus were found in volunteers inoculated with homologous vaccine, lower titers were found in volunteers given A/Port Chalmers or A/England/ 72 vaccine and the lowest levels were seen in volunteers given A/Hong Kong/68 vaccine: the largest number of infections by the challenge virus was seen in volunteers given A/Hong Kong/68 vaccine, less were observed in volunteers given A/England/72 vaccine, and least were found in groups given A/Port Chal- mers or A/Scotland/74 vaccine. Compared with the incidence of infection in volunteers given B/Hong Kong/73 vaccine, all groups given heterologous influenza A vaccines showed some immunity to challenge infection.

Introduction

Immunization with inactivated influenza virus vaccines during interpandemic periods has been shown to induce immunity to challenge with homologous virus in 60-90% of vaccinees (Foy et al., 1971; Hobson, 1975). In addition, inactivated virus vaccines have been reported to induce immunity against heterologous influenza virus infections;

0300--8584/78/0166/0099/~ 02.00

100 C,W. Potter et al.

thus, several studies have reported that immunization with inactivated A/Hong Kong/ 68 vaccine gave protection against infection by influenza A/England/72 (Pereira et al., 1972; Hoskins et al., 1973; Ruben et al., 1974). This finding was probably due to the cross-reactivity of the haemagglutinins of the vaccine and the infecting viruses (Schild et al., 1974), since antibody to the haemagglutinin has been shown to correlate best with immunity to challenge virus infection (Meiklejohn et al., 1952; Hobson et al., 1972).

The above results suggest that vaccines prepared against one strain of virus may be effective in producing immunity against related interpandemic virus strains; however, the extent of cross-protection has not been established. To investigate this prob- lem, groups of hamsters were immunized with different inactivated influenza virus vaccines. All the animals were later inoculated with one of two live influenza virus strains, and the extent of cross-immunity induced by the different vaccines was measured. A similar study was carried out in volunteers; in this study, groups of students were immunized with one of five inactivated influenza virus vaccines. One month later all volunteers were inoculated with WRL 105, and the degree of immunity to challenge infection induced by the various vaccines was assessed.

Materials and Methods

Influenza Viruses

Influenza viruses A/FM/1/47 (H1N1), A/Scotland/74 (H3N2), A/Port Chalmers/73 (H3N2), A/England/72 (H3N2), A/Hong Kong/68 (H3N2), A/Victoria/75 (H3N2), and B/Hong Kong/73 were obtained from Dr. G.C. Schild (National Institute of Med- ical Research, Mill Hill, London). Virus pools were prepared from the allantoic fluid of 10-day embryonated eggs inoculated with 0.2 ml of a 10 -2.0 dilution of seed virus. After 48-h incubation at 33~ the allantoic fluids were harvested and stored at -80oc.

Monovalent, inactivated virus vaccines were prepared from influenza viruses A/Scot- land/74, A/Port Chalmers/73, A/England/72, A/Hong Kong/68, and B/Hong Kong/73 by Evans Biologicals Ltd. The vaccines were standardized to contain 400 IU (inter- national units) per 0.5 ml.

Attenuated influenza virus WRL 105 (H3N2), a recombinant of influenza A/Fin- land/74 (H3N2) and A/Okuda/57 (H2N2), was kindly supplied by Dr. D.S. Freestone (Wellcome Research Laboratories, Beckenham, Kent); the properties of this vaccine have been described previously (Morris et al., 1975; Moffat et al., 1976; Freestone et al., 1976).

Virus Isolation

Lungs from hamsters, collected and processed as described previously (Jennings et al., 1976), and throat swabs from volunteers were collected 3 days after virus infection into PBS-containing 2% bovine serum albumin and antibiotics and stored at -80~ For virus isolation, specimens were thawed and 0.2 ml inoculated by the allantoic route into each of two lO-day embryonated eggs. The eggs were incubated at 33oc for 48 h, after which time the allantoic fluids were tested for virus haemagglutination using fowl cells.

Immunity to Influenza Virus Infection 101

Serological Tests

I. Haemagglutinationqnbibition (HI) Tests: These were carried out by a modification of the microtitre method of Sever (1962), as described previously (Potter et al., 1975). Prior to testing the sera were treated with cholera filtrate (Burroughs Wellcome Ltd.) for 18 h at 37oc, and subsequently heated for 30 min at 56oc.

2. Neuraminidase4nbibition (NI) Tests: NI antibody tests were performed by the automated method based on the standard World Health Organisation technique (Aymard-Henry et al., 1973), as described previously (Bevan et al., 1975). The source of neuraminidase for all the antibody titrations was purified influenza virus A/Scotland/ 74 disrupted with 1.0% triton (Bevan et al., 1975).

3. Complement-fixation (CF) Tests: These tests were carried out by the method of Bradstreet and Taylor (1962) using 2.5 MHD50 of guinea pig complement and A/Port Chalmers/73 virus-soluble antigen extracted from the chorio-allantoic membranes of virus-infected eggs. The tests were incubated overnight at 4 o c prior to the addition of sensitized sheep cells, and the antibody titres were taken as the highest serum dilution that gave 75% or more fixation of complement.

Results

A. Studies in Hamsters

Response to Immunization ~uitb Inactivated Influenza Virus Vaccines: Hamsters were lightly anaesthetized, and inoculated intranasally and dropwise with 107.0 EID50 of influenza virus A/FM/1/47 in an 0.2 ml volume of PBS; this priming procedure was necessary, since unprimed animals do not produce antibody in response to inactivated vaccines (Potter et al., 1974; Jennings et al., 1974). Four weeks after the priming infection, the animals were divided into groups of six, and inoculated i.ntramuscularly with 200 IU of inactivated A/Scotland/74, A/Port Chalmers/73, A/England/72, A/Hong

Table 1. Serum HI antibody response of hamsters a to immunization with inactivated influenza virus vaccines

Inactivated vaccine

given No. (200 IU) tested

Serum HI antibody response to vaccine virus

No. developing Pre - immune Post - immune Fold serum antibody titre (gmt) titre (gmt) increase

A/Scotland/74 6 6 /6

A/Port Chalmers/73 8 6/8

A/England/72 6 5/6

A/Hong Kong/68 6 4 / 6

B/Hong Kong/73 6 O/8

5,0

5.0

5.0

5.0

5.0

24.9

40,8

13.8

12,8

5.0

5,0

8,2

2.8

2.6

1.0

aAnimals primed by infection with A/FM/1/47 virus 4 weeks prior to immunization

102 C.W. Potter et al.

Kong/68, or B/Hong Kong/73 virus vaccines. Three weeks later the animals were bled and the serum HI antibody response to immunization measured. The results are shown in Table 1. Most animals produced serum HI antibody in response to immunization. Thus, all animals given A/Scotland/74 or A/Port Chalmers/73 vaccines produced antibody and the fold-increase in titre was 5.0 and 8.2, respectively. Not all animals produced serum HI antibody following immunization with A/Hong Kong or A/England vaccines, and the mean titres were relatively low (Table 1). Animals given B/Hong Kong/73 virus vaccine did not produce homologous serum HI antibody; previous studies have shown that a priming infection with an influenza A virus does not poten- tiate an antibody response to an influenza B vaccine (Potter et al., 1973).

Susceptibility of Immunized Hamsters to Challenge Virus Infection: The titres of serum H! antibody to A/Scotland/74 virus produced by immunization with homologous and heterologous inactivated influenza A virus vaccines are shown in Table 2. All hamsters given A/Scotland/74 virus vaccine produced homologous antibody; the gmt increased five-fold to 24.9. Serum HI antibody to A/Scotland virus was also found following immunization with A/Port Chalmers/73 and A/England/72 virus vaccines; for these hamsters, the increase in gmt was 2.7 and 2.0, respectively. In contrast, immunization of primed hamsters with A/Hong Kong/68 vaccines did not induce antibody to the A/Scotland/74 virus.

Three weeks after immunization, three animals in each of the vaccine groups were inoculated intranasally with 107.0 EID50 of A/Scotland/74 in an 0.2 ml volume of PBS. Three days after inoculation, the lungs were removed and extracts tested for infectious virus in lO-day embryonated eggs. The results showed that virus was not recovered from any of the hamsters previously immunized with A/Scotland/74, A/Port Chalmers/73, or A/England/72 virus vaccine (Table 2). In contrast, lung extracts from two of three animals previously given A/Hong Kong/68 vaccine and all three animals previously given B/Hong Kong/73 vaccine contained relatively high titres of infectious virus 3 days after infection.

Table 2. Infection of hamsters with A/Scotland/74 virus following immunization with inactivated vaccines

HI antibody titres to A/Scotland/74

Vaccine No. Pre - immune given tested titres (gmt )

A/Scotland/74 6 <10 (5.0)

A/Port Chalmers/73 6 <10 (5.0)

A/England/72 6 < 10 (5.0)

A/Hong Kong/68 6 <10 (5.0)

B/Hong gong/73 6 <10 (5.0)

Post - i m m u n e titres (gmt )

15-40 (24.9)

<10-30 (13.3)

<10-20 (9 .7)

<10 (5.0)

<10 (5.0)

Fold increase

5.0

2.7

2.0

1.0

1.0

Infection with A/Scotland/74 "

0 /3

0 /3

0 /3

2 /3

3 /3

* Infection determined by recovery of virus from lungs collected 3 days after virus infection


Table 3. Infection of hamsters with A/Hong Kong/68 virus following immunization with inactivated vaccine

Vaccine given

A/Scotland/74

A/Poet Chalmers/73

A/England/72

A/Hong Kong/68

B/Hon 9 Kong/73

No. tested

HI antibody titres to A/Hong Ko~g/68

Pre - immune titres (gmtl

6 <10 (5.0}

6 <10 (S.O)

6 <10 (5.0)

6 <10 (5,0)

6 <10 (5.01

Post - immune titres (gmt)

<10 (5.0)

<10 (5.0)

<10-30 (8.9)

<10-30 (12.81

<10 (5.0)

Fold increase

1.0

1.0

1.8

2.6

1.0

Infection with A/Hon 9 Kong/68 *

2/3

2/3

0 /3

0 /3

3 /3

* Infection determined by virus isolation from lungs collected 3 days after virus inoculation

The serum specimens from hamsters immunized with the various influenza A vaccines were also titrated for HI antibody to A/Hong Kong/68 virus. The results are shown in Table 3. The serum HI antibody titres against A/Hong Kong/68 virus in hamsters given homologous vaccine increased from gmt 5.0 to 12.8 following immunization. Hamsters given A/England/72.vaccine also produced antibody to the A/Hong Kong virus; the titre increased from 5.0 to 8.9 (gmt) following immunization. Immuni- zation of hamsters with A/Scotland/74 or A/Port Chalmers/73 virus, however, did not induce antibody to the A/Hong Kong/68 virus (Table 3). Three weeks after immunization, three animals in each group were inoculated intranasally with 107.0 EID50 of A/Hong Kong/68 virus in an 0.2 ml volume of PBS; infection with this challenge virus was determined by virus isolation from lung homogenates collected 3 days after virus infection. Virus was recovered from all hamsters given B/Hong Kong/73 vaccine, from two of three hamsters given A/Scotland/74, and from the same number of animals given A/Port Chalmers/73 vaccine. In contrast, virus was not recovered from hamsters previously immunized with homologous A/Hong Kong/68 vaccine or heterologous A/England/72 virus vaccines (Table 3).

B. Studies in Volunteers

Response of Volunteers to Immunization with Inactivated Influenza Virus Vaccines: Volunteers were test bled and immunized with 400 IU of one of five inactivated influenza virus vaccines; a second blood specimen was collected 4 weeks later. The serum HI antibody response to homologous vaccine is shown in Table 4. The best HI antibody response was seen in volunteers given A/England172 vaccine; following immunization with this vaccine the gmt increased 16.6-fold to 812.0. In contrast, the worst response was for volunteers given A/Hong Kong/68 vaccine; immunization in these volunteers produced only a 4.2-fold increase in HI antibody titre. The response to immunization with A/Port Chalmers/73, B/Hong Kong/73, and A/Scotland/74 virus vaccines gave results which were between those obtained with A/England/72 and


Table 4. Serum HI antibody response to immunization

Inactivated vaccine

given (4O0 IU)

A/Scot land/74

A/Por t Cha liners/73

A/England/72

A/Hong Kong/68

B/Hong Rong/73

No. tested

27

25

26

25

31

Serum HI antibody response to vaccine virus

No. 4 - fold increase

(%)

22 (81.)

14 (56)

24 (92l

11 (44)

18 (58)

Pre - immune tltre (gmt)

15.0

39.1

48,8

81.2

6.1

Post - immune titre (gmt)

238.8

256.3

812,0

340.0

28.3

Fold increase

15.9

6.6

16.6

4.2

4.6

A/Hong Kong/68 vaccines. The variable serum HI antibody response to immunization was not due entirely to the variable levels of homologous, pre-immune antibody; the results show no correlation between the titres of pre-immune antibody and the fold- increase in antibody titre found after immunization (Table 4). These results suggest that the different influenza virus vaccines may differ considerably in their ability to induce serum HI antibody.

Serum HI Antibody Response to A/Scotland/74 Virus: The serum HI antibody response of volunteers to influenza virus A/Scotland/74 following immunization with different inactivated influenza virus vaccines is shown in Table 5. The titres of antibody present prior to immunization were similar in the five groups; the gmt varied from 10.8 for volunteers given A/England/72 vaccine to 20.7 for those given A/Port Chalmers vaccine. The postimmunization titres of antibody to A/Scotland/74 varied considerably. For volunteers given the homologous A/Scotland vaccine, the serum HI antibody titre increased 15.9-fold to 1:239. The best heterologous HI antibody response was found in volunteers immunized with A/Port Chalmers/73 and the worst was with A/Hong Kong/68;

Table 5. Serum HI antibody response to A/Scotland/74 virus

Inactivated vaccine given No.

{400 IU) tested

A/Scotland/74 27 1

A/Port Cha Imers/73 25 2

A/England/72 26 4

A/Hong Kong/58 25 14

B/Hong Kong/73 31 19

HI titre (post-immune) to A/Scotland/74

<20 20 -40 60 -120 >120

1 3 22

2 10 11

7 7 8

2 5 4

5 6 1

Changes in gmt (fold increase)

15.0 - 239 (15.8]

20.7 - 119 (5.7)

10.8- 63 ( 5.8)

11.6- 25 ( 2.1'}

14.7-15.0 (1.O)


the haemagglutinins of these two viruses are, respectively, the most and least cross-re- active haemagglutinins with A/Scotland virus used in the present study (Schild et al., 1974).

The distribution of serum HI antibody titres to influenza virus A/Scotland/74 following immunization are also shown in Table 5. The antibody titres are placed in four groups to correlate with the titres which have been reported to give immunity to challenge virus infection (Meiklejohn et al., 1952; Hobson et al., 1972); these studies have shown that subjects with serum HI antibody titres of ~> 40 exhibit i> 50% immunity to virus infection, whilst titres below this level gave < 50% immunity. Thus, of 27 volunteers given A/Scotland vaccine, only 2 (7%) had serum HI antibody titres of < 40 to the homologous virus after immunization and are presumed to be susceptible to the challenge virus. The results for the volunteers in the other groups were 4 of 25 (16%), 11 of 26 (42%), 16 of 25 (64%), for volunteers given inactivated influenza A/ Port Chalmers/73, A/England/72, and A/Hong Kong/68, respectively.

Response of Volunteers to Cballenge Infection: Four weeks following immunization, all the volunteers were inoculated intranasally and dropwis.e with 107.0 EIDs0 of attenuated influenza virus WRL 105 in an 0.5 ml volume. Infection by the challenge virus was determined either by virus recovery from throat swabs taken 3 days after infection or by a four-fold or greater increase in serum HI antibody titre to A/Finland/ 74 virus. Rises in antibody were determined by both HI and CF tests, and a >/4-fold increase in antibody by either test was taken as proof of virus infection (Potter et al., 1975). The results are shown in Table 6. Evidence of infection was obtained for one volunteer previously immunized with A/Scotland/74, and for one volunteer previously immunized with A/Port Chalmers/73 vaccine. Thus, evidence of infection by WRL 105 virus was approximately the same for volunteers given A/Scotland/74 or A/Port Chal- mers/73 vaccine (Table 6). For volunteers given A/England/72, A/Hong Kong, or B/ Hong Kong/73, evidence of virus infection was obtained for 2 of 26 (8%), 7 of 25 (28%) and 14 of 31 (48%), respectively. Compared with the results obtained for volunteers

Table 6. Infection by WRL 105 virus in volunteers previously given inactivated vaccine

Vaccine No. given tested

A/Scotland/74 27

A/Port Cha liners/73 25

A/England/72 26

A/Hong Kong/68 25

B/Hong Kong/73 31

HI antibody titre *

30 (%) > 40 (%)

2 25

2 23

8 18

16 9

24 7

Infection with WRL 105

Virus isolation

Significant HI/CF

antibody response

1

2

S

14

Tota I (%1

1 ( 4 )

1 ( 4 l

2 ( 8 )

7 (28)

14 (48)

* Serum HI antibody titre to WRL 105 virus following immunization and prior to challenge infection


given B/Hong Kong/73 vaccine, which served as a control group, measurable immunity to challenge infection for WRL 105 was found following immunization with all the influenza A viruses tested, and the infected volunteers in each group were a similar proportion of those with low serum HI antibody titres. Thus, the degree of immunity was variable; the most solid immunity was induced by influenza A/Scotland and A/Port Chalmers vaccine, and the least significant results were obtained for volunteers immunized with A/Hong Kong vaccine (Table 6).

Discussion

The rapid antigenic drift of influenza H3N2 viruses that has occurred during the past few years (Schild et al., 1974) has resulted in some instances in which immunization has been carried out with virus strains which are antigenically distinct from the current infecting virus. Observations from these occurrences showed that one heterologous vaccine induced significant immunity; thus, immunization with inactivated A/Hong Kong/68 vaccine conferred significant immunity against subsequent infection by A/ England/72 virus (Pereira et al., 1972; Ruben et al., 1974). The present studies were carried out to investigate further the extent of immunity induced by heterologous influenza virus vaccines, since the published examples of heterologous immunity describe immunity to A/England/72 infection following immunization with A/Hong Kong vaccine and the phenomenon might relate to these two viruses only.

The response of both animals and volunteers to the different influenza A vaccines varied widely. In hamsters the highest serum HI antibody titres were seen following immunization with A/Port Chalmers virus and the worst following immunization with A/Hong Kong; in contrast, the best HI antibody response of volunteers was found following immunization with A/England/72 virus and A/Scotland/74 vaccine, and the worst following immunization with A/Hong Kong/68 vaccine. Although the level o f homologous pre-immune antibody affects the response of volunteers to immunization with inactivated virus vaccine, there was no clear correlation between the level of pre- immune antibody and response to immunization. The results indicate that the different strains of H3N2 viruses differ in their ability to induce serum H1 antibody.

Cross-HI tests have shown that the influenza H3N2 viruses have each year exhibited increased antigenic drift from the original A/Hong Kong/68 strain (Schild et al., 1974). The ability of various inactivated vaccines to induce serum HI antibody to A/Scotland/ 74 virus and to immunize hamsters against infection by this virus was directly related to the cross-reactivity between the various strains. In addition, the ability of the same vaccines to induce antibody and to immunize against infect ionby A/Hong Kong/68 virus also diminishes in the same manner. A similar study in volunteers shows the same result. Immunization with A/Scotland/74 vaccine produced most significant immunity to challenge infection with WRL 105 ; the surface antigens of this virus are very closely related to the A/Scotland/74 virus (Freestone et al., 1976). A similar immunity to infection by WRL 105 virus was seen following immunization with A/Port Chalmers/73; of the vaccine viruses tested, this virus has the greatest degree of cross- reactivity with A/Scotland/74. Immunization of volunteers with A/England/72, which shows significant drift from A/Scotland, produced significantly less serum HI antibody to A/Scotland virus and a lower level of immunity to challenge infection;


immunization with A/Hong Kong/68 produced the least amount of cross-reacting antibody with A/Scotland virus, and the lowest level of immunity to challenge virus infection. However, it was noted that the demonstrable immunity was achieved following immunization with all four H3N2 viruses used. The present results, together with the published reports of immunity to A/England/72 infection following immunization with A/Hong Kong vaccine, indicate that immunity can be accomplished by immunization with heterologous cross-reacting influenza virus vaccines. The results also show that the heterologous vaccines do not induce antibody to the same titre as homologous virus vaccine, and it would be expected, therefore, that the heterologous immunity would be shorter lived. However, since the influenza H3N2 viruses have shown antigenic drift each year, the immunity induced by some heterologous virus vaccines could be sufficient. Thus, the A/Port Chalmers/73 vaccine could have been used as effectively as homologous virus vaccine for immunization against the A/Scotland/74 infection, and the shorter duration of immunity would not have been a practical prob- lem since the A/Victoria/75 virus caused epidemic infection the following year.

References

Aymard-Henry, M., Coleman, MIT., Dowdle, W.R., Laver, W.G., Webster, R.G.: In- fluenza virus neuraminidase and neuraminidase-inhibition test procedures. Bull. WHO 48,199--202 (1973)

Bevan, A., Furminger, I.G.S., Smith, C.H.: Neuraminidase assay of influenza vaccines. Dev. Biol. Stand. 28 ,173-179 (1975)

Bradstreet, C.M.P., Taylor, C.E.D.: Technique of complement-fixation applicable to the diagnosis of virus diseases. Month. Bull. Min. Hlth. Lab. Serv. 21 ,96-104 (1962)

Foy, H.M., Cooney, M.K., McMahan, R., Bor, E., Grayston, J.T.: Single-dose monovalent A2/Hong Kong influenza vaccine. J.A.M.A. 217, 1067--1071 (1971)

Freestone, D.S., Bowker, C.H., Letley, E., Ferris, R.D., White, W.G., B~rnes, G.M.: A clinical trial of WRL 105 strain live attenuated influenza vaccine comparing four methods of intranasal vaccination. J. Hyg. (Camb.) 76 ,459-466 (1976)

Hobson, D.: Assessment of the efficacy of influenza vaccines against natural challenge. Dev. Biol. Stand. 28,285-.294 (1975)

Hobson, D., Beare, A.S., Ward-Gardner, A.: Haemagglutination-inhibiting serum antibody titres as an index of response of volunteers to intranasal infection with live, attenuated strains of influenza virus. Proceedings of the Symposium on Live In- fluenza Vaccines, Zagreb, 1971. pp. 73-84 Yugoslav Acad. of Sci. Art. (1972)

Hoskins, T.W., Davies, J.R., Allchin, A., Miller, C.L., Pollock, T.M.: Controlled trial of inactivated influenza vaccine containing A/Hong Kong strain during an outbreak of influenza due to the A/England/42/72 strain. Lancet 1973 II, 116--120

Jennings, R., Denton, M.D., Potter, C.W.: The hamster as an experimental animal for the study of influenza. 1. The role of antibody in protection. Med. Microbiol. Immunol. 162,217-226 (1976)

Jennings, R., Potter, C.W., MeLaren, C.: Effect of preinfection and preimmunization on the serum antibody response to subsequent immunization with heterotypic influenza vaccines. J. Immunol. 113, 1834-1843 (1974)

108 c.w. Potter et al.

Meiklejohn, G., Kempe, C.H., Tahalman, W.G., Lennette, E.H.: Evaluation of monovalent influenza vaccines. Observations during an influenza A-prime epidemic. Am. J. Hyg. 55, 12-21 (1952)

Moffatt, M.A.J., Stealey, V.M., Freestone, D.S., MacDonald, A.: Assessment of elicited antibody responses, clinical reactions and transmissibility of WRL 105 live influenza vaccine. J. Biol. Stand. 4, 92-95~ (1976)

Morris, C.A., Freestone, D.S., Stealey, V.M., Oliver, P.R.: Recombinant WRL 105 strain live attenuated influenza vaccine. Laneet 1975 II, 196--199

Pereira, M.S., Chakraverty, P., Schild, G.C., Coleman, M.T., Dowdle, W.R.: Prevalence of antibody to current influenza viruses and effect of vaccination on antibody response. Brit. Med. J. 1972 IV, 701-703

Potter, C.W., Jennings, R., McLaren, C., Edey, D., Stuart-Harris, C.H., Brady, M.: A new surface-antigen-adsorbed influenza virus vaccine. II. Studies in a volunteer group. J. Hyg. (Camb.) 75 ,353-362 (1975)

Potter, C.W., Rees, R.C., Shore, S.L., McLaren, C.: Immunity to influenza in ferrets IX Delayed hypersensitivity following infection or immunization with A2/Hong Kong virus. Microbios. 10, 7--21 (1974)

Ruben, F.L., Johnston, F., Strciff, E.J.: Immunity in a partially immunized aged pop- ulation. Effectiveness of killed Hong Kong vaccine against infection with the Eng- land strain. J.A.M.A. 230, 863--866 (1974)

Schild, G.C., Oxford, J.S., Dowdle, W.R., Coleman, M.T., Pereira, M.S., Chakraverty, P.: Antigenic variation in current influenza A viruses. Evidence of a high frequency of antigenic drift in the Hong Kong virus. Bull. WHO 51, 1--11 (1974)

Sever, J.L.: Application of a microtechnique to viral serological investigations. J. Immunol. 88, 320-329 (1962)

immunity to influenza virus infection induced by heterologous, inactivated vaccines

Documents