Variable efficacy of repeated annual influenza vaccination

Repeat Vaccination Was Beneficial When Given to Previous Vaccinees.

In all groups that received v1, it always lowered experimental attack rates to receive v2. The lowering of attack rates was statistically significant, P < 0.01, other than in the group in which the v1–v2 distance was 0 and the v1–e distance was 2 (P < 0.05), and in the group in which the v1–v2 distance was 2 and the v1–e distance was 0. This can be seen by comparing the experimental attack rates in the v1-only column of Table 2 with the attack rates in the corresponding row of the repeat-vaccine columns (a row corresponds to groups in which the v1–e distance was the same).

Negative and Positive Interference of a Prior Vaccine.

Attack rates for repeat vaccinees were sometimes lower, and sometimes higher, than the attack rate for first-time vaccinees (Table 2)—even though the timing, dose, and v2–e distance were identical. The effect of v1 on the apparent efficacy of v2 can be split into two factors: negative and positive interference. Because of the first vaccination, v2 is partially eliminated by preexisting cross-reactive antibody produced in response to v1 and by v2 stimulating v1 memory clones. These effects, which lead to reduced effectiveness of v2, we call “negative interference.” Negative interference is greater when the v1–v2 distance is small.

The second factor influencing the attack rate in repeat vaccinees is “positive interference,” in which preexisting cross-reactive antibody produced in the response to v1, and boosted by the response to v2, helps to clear the epidemic virus. Positive interference is greater when the v1–e distance is small—this trend can be seen in the columns of Table 2.

Fig. 3a shows an example of negative interference of v1 on v2. The response to v1 reached levels** typical for the model. The response to v2 caused a 6-fold increase in the antibody cross-reactive with v1. This increase was caused by stimulation of cross-reactive memory B cells produced in the response to v1 (data not shown). After the response to v2, the antibody reactive with v2 was approximately one-half of that usually seen after a primary vaccination (e.g., the level in the response to v2 in Fig. 3b, and the response to v1 in Fig. 3 a and c). Furthermore, the antibody produced in response to v1 and v2 was strongly cross-reactive with v1, but only weakly cross-reactive with the epidemic strain. This absence of protective antibody cross-reactive with the epidemic virus allowed the virus to exceed the 1500 antigen unit threshold and “cause disease” (Fig. 3a). The antibody level reactive to the epidemic strain was large after the epidemic virus was cleared, though the antibody levels to v1 and v2 remained mostly unaffected (Fig. 3a), suggesting a de novo immune response to the epidemic strain. The effects of negative interference are most evident in comparison with the response generated by the v2-only control (Fig. 3b). The antibody produced in this case cross-reacted with the epidemic strain and provided protection against the epidemic challenge.

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Figure 3

An example of negative interference by v1 on v2 (a), a v2-only control for comparison (b), and positive interference by v1 on the epidemic challenge (c). The v1–v2 distance was 2 for the examples in a and c, and the v1–e distance was 4 for the example in a and 2 for the example in c. An enlarged region of a is included in the supplemental material (www.pnas.org).

Fig. 3c shows an example of positive interference of v1 on the epidemic challenge. As in Fig. 3a, the response to v1 was typical, and v1 negatively interfered with v2 by about the same amount (the v1–v2 distance was the same in both cases). The difference between this example and Fig. 3a is that the antibody produced in response to v1, and boosted by v2, was cross-reactive with the epidemic strain, whereas it was not cross-reactive in Fig. 3a (the v1–e distance is 2 in Fig. 3c and 4 in Fig. 3a). The antibodies cross-reactive with the epidemic strain were sufficient in number and affinity to clear the epidemic virus before it caused disease. Because the epidemic virus was cleared quickly, it did not stimulate the production of new antibody and the antibody levels to all three antigens was mostly unaffected by the challenge.

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Figure 4

Homologous vaccination over a range of v–e distances. (a) Comparison of giving the same vaccine in the prior influenza season, the current season, and both seasons. (b) Observed vaccine efficacy in repeat vaccinees relative to the efficacy in first-time vaccinees and predicted efficacy based on the antigenic distance hypothesis. Observed data are from the four outbreaks in the Hoskins and Keitel studies preceded by homologous vaccination [1972, 1984, 1985, and 1987 (B)]. The predicted vaccine efficacy is computed from panel a as 1 − attack rate. In computing the efficacy relative to first-time vaccinees we assumed that first-time vaccinees received the vaccine in the current season.

For some repeat vaccine groups, positive interference dominated over negative interference (entries marked with an ∗ or ∗∗ in Table 2) and the attack rate was lower than for first-time vaccinees. In other repeat vaccine groups negative interference dominated (entries marked with a † or ‡ in Table 2) and the attack rate was higher than for first-time vaccinees.

When the v1–e distance was less than 4, a greater proportion of memory B cells that cross-reacted with the epidemic strain were initially produced in response to v1 than to v2 (data not shown). This observation, the result of negative interference, was more pronounced when the v1–v2 distance was small.

When the v1 and v2 strains were the same, there was a high attack rate (0.78) for repeat vaccinees (Table 2, v1–v2 distance = 0). This finding was investigated further with experiments in which the v–e distances were 0, 1, and 3. In these homologous vaccination experiments, repeat vaccinees had higher attack rates than corresponding first-time vaccinees and lower attack rates than corresponding v1-only groups (Fig. 4a).

Historical Analysis.

The Hoskins and Keitel studies reported vaccine and epidemic strains, together with vaccine efficacies in first-time and repeat vaccinees, for each outbreak (Table 3); thus, they can be used to test the antigenic distance hypothesis. Field study data were accepted according to the definitions in ref. 3: first-time vaccinees were defined as individuals vaccinated immediately before the influenza season in which an outbreak occurred, and not vaccinated in the preceding years; repeat vaccinees were defined as individuals vaccinated immediately before the influenza season in which an outbreak occurred, and also vaccinated in one or more consecutive preceding years. Thus for our study, the Hoskins 1976 (H3N2) outbreak was excluded because there was no strict first-time vaccine group (6), and the outbreaks in the first year (1983) of the Keitel study were excluded because there was no strict repeat vaccine group. The antigenic distances among the vaccine and epidemic strains were determined from historical HI assay tables‡‡ kindly provided by the Centers for Disease Control and Prevention, Atlanta. Differences in HI values were scaled to equate them with our measure of antigenic distance†† and are shown in Table 3. Using our model and the HI derived antigenic distances, we predicted the vaccine efficacy for first-time and repeat vaccinees. No attempt was made to tune parameters of the model to fit the Hoskins and Keitel observations.

The predictions from the model of vaccine efficacy in repeat vaccinees relative to the efficacy in first-time vaccinees had good correlation (r = 0.87) with that seen in the Hoskins and Keitel studies (Table 3, Figs. 1 and 5). However, the model did not accurately predict absolute vaccine efficacies. Nor was there any correlation between vaccine efficacy in first-time vaccinees and the v2–e distance.

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Figure 5

Predicted and observed vaccine efficacy in repeat vaccinees relative to the efficacy in first-time vaccinees for the outbreaks during the Hoskins and Keitel studies. Correlation 0.87 (P = 0.01); linear regression slope 1.03, intercept 0.07.

Homologous Vaccination.

Intuitively, it seems wrong that repeat vaccinees given the same (homologous) vaccine for two successive years could be less protected than first-time vaccinees. However, the model predicts that as the v–e distance increases from 0 to 2, the ratio of vaccine efficacy in repeat vaccinees compared with that in first-time vaccinees will fall monotonically from 1.0 to 0.5. This prediction holds for the four outbreaks in the Hoskins and Keitel studies that were preceded by homologous vaccination (Fig. 4b). Homologous vaccination can result in low vaccine efficacy because it causes maximum negative interference of the first vaccine on the second vaccine, and thus repeat vaccinees are protected only by the positive interference of the first vaccine on the epidemic strain (which depends on the v–e distance).

In two other cases of homologous vaccination [1976 (H3N2) and 1986 (H1N1)], the v–e distance was 2, and thus the model would have predicted poor efficacy in repeat vaccinees. However, in both cases, a second nonhomologous v2 strain was also given and the model predicted approximately equal efficacy in first-time and repeat vaccinees (a good prediction for the 1986 outbreak, and not testable for the 1976 outbreak).

Failure of homologous strain vaccination to boost after 2 weeks (11) and after 6 months (12) has been seen in human trials. We have previously performed simulations, similar to those reported here, but in which antibody levels fell close to zero levels before revaccination (13). In those experiments a homologous revaccination provided good protection. Thus, the results of the model depend on the persistence of antibody and whether that persistence remains above protective levels.

Negative interference of a prior vaccine on a subsequent vaccine has been seen in influenza vaccination in humans (14, 15), as has the preferential production of antibody cross-reactive with the prior vaccine (15, 16). As in the model, the amount of vaccine antigen elimination by preexisting cross-reactive antibody appears to be proportional to the amount of circulating antibody (11, 17).

Historical Analysis.

The ability of the model to predict the ratio of repeat to first-time vaccine efficacies, yet not predict absolute vaccine efficacies, suggests that taking the ratio cancels out a linear factor that affects first-time and repeat vaccinees equally—a factor that has been obscuring the relationship between antigenic distance and vaccine efficacy. This factor (or factors) could be caused by variation in the antigenicity or dose of the vaccine—factors not explored in the current study.

Although the results from the two Keitel study outbreaks in 1983 were excluded from the statistical testing of the hypothesis, the differences between the predicted and observed ratios for these outbreaks support the hypothesis. The outbreaks were excluded because although repeat vaccinees received v1 sometime within the 3 years preceding the study, not all of them had received v1 in the fall of 1981. This would allow antibody levels to decrease before vaccination in the fall of 1982. Lower antibody levels would decrease the negative interference of v1 on v2, and thus favor repeat vaccinees (as was seen). The extreme 4-fold favoring of repeat vaccinees in the 1983 (H1N1) outbreak also supports the hypothesis: The vaccine (A/Brazil/11/78) appears to have been of low antigenicity (observed vaccine efficacy in first-time vaccinees was only 21%) and antibody persistence was less than for concomitant H3N2 and B vaccination (12). Thus negative interference would be likely to be low, and a repeat homologous vaccination would produce a more classic secondary response with high antibody titers, and thus be highly effective (as was seen).

In the Hoskins study it was found that boys who had confirmed influenza infection were better protected against subsequent outbreaks than boys who had not been infected (but who had been vaccinated). The results from the model offer a potential explanation. In the model, antibody levels after the epidemic challenge were different, depending on whether the experimental challenge caused disease. If the challenge caused disease, the antibody levels cross-reactive with the epidemic strain were at least 3-fold higher than after a primary or secondary vaccination (Fig. 3a)—in the model this was due to the antigen stimulating the immune system for a longer time, leading to more plasma cells and antibodies being produced. Thus, antibody levels to the epidemic virus remained higher for longer, and would be protective for longer, than the lower antibody levels in response to vaccination. Conversely, when the epidemic virus was cleared quickly it had little effect on antibody levels (Fig. 3 b and c); thus, we would expect it to have little influence on future protection.

In the model, repeat vaccinees received only two vaccinations, and were exposed to a single experimental epidemic challenge after the second vaccination. However, in both the Hoskins and Keitel studies, some repeat vaccinees were vaccinated more than twice, and individuals might have been exposed to epidemic challenge multiple times; yet the model still made good predictions. This observation suggests that the last two vaccinations are the most important in determining efficacy in repeat vaccinees.

Vaccine Design.

One focus of influenza vaccine strain selection is the selection of a strain that will be close to the (unknown) upcoming epidemic strain. The antigenic distance hypothesis supports this strategy, and adds the following: If there is a choice among otherwise equivalent strains (i.e., strains thought to be equally good guesses of the upcoming epidemic strain and equally appropriate for manufacture), then choosing the strain farthest from the previous vaccine strain would reduce the effects of negative interference and thus potentially increase vaccine efficacy in repeat vaccinees.

Vaccine design strives for persistent high antibody titer and persistent cellular memory. However, this may not be the optimal strategy if the pathogen is antigenically variable and revaccination is necessary. In the case of annual influenza vaccination, if the antibody titer is high for more than a year then it has the potential to negatively interfere with a subsequent revaccination. One option to overcome negative interference would be to increase the vaccine dose; although this could work for one influenza season, the strategy has the danger of resulting in high antibody levels that would increase negative interference in subsequent years. An alternate, counter-intuitive, option would be to design vaccines in which the vaccine specific antibody titer remained high only during the influenza season, and then decreased substantially before revaccination, thus lowering the potential negative interference on subsequent vaccinations.

Skipping vaccination for a year would be another method to allow antibody titers to fall and thus reduce negative interference in the following year. However, while skipping vaccination might benefit repeat vaccinees in the following year, they would suffer in the year they skipped because, even though some repeat vaccination groups in our experiments had significantly higher attack rates than the first-time vaccination group, they still had a lower attack rate than groups that skipped the second vaccination (Table 2).

Summary.

The antigenic distance hypothesis, as tested by comparing the predictions from a computer model to seven influenza outbreaks observed in the Hoskins and Keitel studies, accurately predicted the year-to-year variation in vaccine efficacy of repeat vaccinees relative to that in first-time vaccinees. The hypothesis suggests considering antigenic distance to the previous year’s vaccine strain as part of the vaccine strain selection process, a refinement that has the potential to increase the vaccine efficacy in repeat vaccinees. The methods presented here may also have application to vaccination strategies for other antigenically variable pathogens requiring multiple vaccinations for protection.