1 Introduction infrequent such that by the time a novel strain has risen to appreciable levels in the population, the epidemic 90 wave by the previous strain has largely concluded. In the first instance we will assume that antigenic change 91 occurs without considering its origin (i.e., whether it is from mutation or imported). We will later discuss 92 the implications of these assumptions for our results. 93 Let S 0 denote the initial density of individuals without infection-acquired immunity (susceptibles); note 94 that population densities have been normalized such that S 0 = 1 implies that every individual in the popu-95 lation is susceptible. S 1 is then the density of susceptibles after the first wave (caused by the initial strain 96 A 0 B 0 ) and in general S n denotes the susceptible density following the n th wave. These individuals can be 97 further divided based upon whether they have been vaccinated. Let U n denote the density of susceptible, in successive waves (which we denote ∆ ); in this example, these distances would be ∆ 0 = 1, ∆ 1 = 1 and 106 ∆ 2 = 0. The final density of individuals vaccinated against A 1 is:
and similarly for the three analogous variables, V A0 n , V B0 n , and V B1 n (see Sup. Info. 5.1).
In general, U n , V kj n , and S n will depend upon which of the 2 n−1 sequences of antigenic changes occurred.
the fraction of susceptibles declines over successive waves. Using again the sequence A 0 B 0 → A 0 B 1 → A 1 B 1 as an example, the potential for a third wave, R
(3) 0 , depends on the overall level of vaccine protection against 117 A 1 B 1 in the population after the second wave:
(3)
Using the same logic, a general formula for R and strain A 2 B 1 or A 1 B 2 emerges. From the preceding assumptions (see Sup. Info. 5.2 for a full derivation), 121 we find that U n is the solution of
Once U 1 , ..., U n are computed using equation (4), we can calculate S n (using equations (1) and (2)). Although 123 our derivation applies for any shape of cross-protection, in what follows, we will assume limited cross-124 protection between variants, χ z ≈ 0 for z > 0. We will discuss the implications of this assumption for our 125 results later. 
In the second scenario, the sequence of epidemic waves is not limited by antigenic change. Specifically, 138 following the initial potential wave by strain A 0 B 0 , the pathogen will undergo successive one unit antigenic 139 changes (and successive potential waves) until the density of remaining individuals without infection-acquired 140 immunity is less than or equal to 1 R; at this point, the population will have herd immunity and so any novel 141 strains will be unable to cause an epidemic wave. In this case, E[S final ] is the average remaining uninfected 142 individuals computed over all possible antigenic sequences terminating in S n ≤ 1 R. For example, if we were 143 to vaccinate exclusively against the primary variant of target A, such that x = 1 and y A = 1, then under the 144 assumption of limited cross-protection (χ z ≈ 0 for z > 0), we find that S n ≤ 1 R the first time the variant 145 A 1 appears. Therefore to compute E[S final ], we would take the average over all possible epidemic sequences 146 ending with a change from A 0 → A 1 (e.g., i.e., protection afforded against the vaccine strain. Case I occurs when protection is sufficiently weak, or the 167 initial strain sufficiently strong, such that the initial wave reduces the density of susceptibles to below this 168 critical threshold 1 R and no further waves can occur. Specifically, case I occurs when
gives the threshold between cases I and II (see Sup. Info. 5.3). In this case, infection-acquired immunity 170 (due to the large number of individuals who were infected) protects the population against further waves 171 (Fig. 2) . From equation (6), any decrease to pathogen transmissibility (β), duration of carriage (δ), or 172 availability of hosts without natural immunity (S 0 ) will lower the threshold TI II (p).
In case III, in contrast, antigenic change is necessary for an epidemic to occur. Case III occurs when 174 vaccine protection is sufficiently strong that the reproductive number of the initial strain is less than one.
In this situation, antigenic change at target A must occur before the epidemic proceeds. 
Case II occurs when coverage is of intermediate strength, specifically, TI II (p) < χ 0 ≤ TII III (p). In this 179 case, the wave caused by the initial strain is such that S 1 > 1 R, and so antigenic change can generate 180 subsequent waves until S n ≤ 1 R (Fig. 2) . Thus, in case II, antigenic change is possible but not necessary 181 for an epidemic. From equation (7), any decrease to pathogen transmissibility (β), duration of carriage (δ), 182 or availability of hosts without natural immunity (S 0 ) will lower the threshold TII III (p). First, consider distributing equally-effective vaccines between two targets: a fraction x of available vaccine 186 doses are used against target A and 1 − x are used against B. All doses are to be allocated to the primary 187 variant at each target, that is, we will vary x while y A = y B = 1. Because both vaccines offer equal protection 188 against the initial strain A 0 B 0 , the choice of x will have no effect on the size of the initial wave, and will only 189 impact subsequent waves, to which conventional vaccination provides no protection (under our assumption 190 of limited cross-immunity). As a result, any vaccine heterogeneity (0 < x < 1) will be at least as good 191 as conventional vaccination in terms of maximizing S n ; in fact, when antigenic evolution either can cause 192 secondary waves (case II) or is necessary for an initial wave (case III), any variation in targets across vaccine 193 doses, 0 < x < 1, will be superior to the conventional vaccination strategy. 
(See Sup. Info. 5.4). Since our choice of x does not affect U 1 or S 1 , it follows from equation (5) that 203 x * will be independent of ω. In comparison with x = 1 2, this choice of x * will reduce protection against 204 the other sequence. However, for a wide range of parameter space, the average of outcomes across the two 205 possible sequences when x = x * outperforms the expected outcome when x = 1 2, and will always outperform 206 conventional vaccination (Fig. 3 ). Of course, by symmetry, if x * is optimal, then so is 1 − x * (Fig. 3) .
If instead pathogen evolution is not limited by antigenic change, maximizing protection against one 208 sequence of antigenic changes means choosing x so that for one of the sequences, S final = 1 R. Again, 209 this means reduced protection against the other sequence of antigenic change, but the average of these two 210 outcomes, E[S final ], generally outperforms x = 1 2, and always outperforms conventional vaccination. As 211 before, if x * is optimal, then so is 1 − x * (Fig. 3) . Next, we consider distributing equally-effective vaccines between two variants of target A, that is, setting 214 x = 1 and varying y A . Clearly, any y A < 1 will reduce the protection against the initial strain, and so if 215 vaccine coverage is sufficiently weak (case I), all doses should be allocated to the primary variant, y A = 1 216 (conventional vaccination). In case II and III, however, conventional vaccination performs poorly because 217 the population is effectively unvaccinated against any variation at target A (Fig. 2) . Therefore we might 218 expect that some doses should be allocated to provide protection against the secondary variant.
If evolution is severely limited by antigenic change (ω → 0), then it is optimal to maximize protection 220 against the primary variant, A 0 ; we denote this choice y A0 . In case II, because conventional vaccination is 221 not sufficiently strong to block the initial A 0 B 0 epidemic wave, we should simply maximize protection against 222 this strain by choosing y A0 = 1. In case III, because conventional vaccination is more than strong enough to 223 completely block the A 0 B 0 epidemic wave, we can divert vaccine doses toward A 1 B 0 . In particular, we can 224 choose y A such that the reproductive number for the first wave is reduced to one, i.e., y A0 satisfies R Fig. 4a ; see also Sup. Info. 5.5).
When the likelihood of antigenic change is high (ω → 1), then so is the threat of a potential second wave 229 by strain A 1 B 0 . In this circumstance it is optimal to maximize protection against the secondary variant at 230 target A, that is, given the next strain is A 1 B 0 , choose y A1 to satisfy R because the initial wave is more likely to deplete the remaining uninfecteds, S 1 , such that further waves pose 236 little additional threat. Note as well that the switch between y A0 and y A1 is sensitive to the level of vaccine 237 coverage, and so for a given ω and RS 0 , y A1 may be favoured in some regions of case II or case III whereas 238 y A0 may be favoured in others; but in general, the closer we are to TI II (p), the more likely y A1 is favoured.
When evolution is not limited by antigenic change (and so is limited by infection-acquired immunity), we This can be achieved with two values of y A : when we choose y A = y * ,
which provides sufficiently weak protection against the initial strain such that S 1 = 1 R (Sup. Info. 5.5).
Second, we can choose y A = y • , where y • > y * such that only after antigenic change at target A will 245 E[S final ] = 1 R (Fig. 4b,e ). Although both y * and y • maximize E[S final ], from a public health perspective 246 y • is the superior option as it maximizes E[S final ] after at least one antigenic change, and so two waves, rather 247 than one epidemic and no antigenic change. By staggering the waves, the peak burden on the healthcare 248 system will be reduced (e.g., [21] ). Indeed, the less desirable outcome of y * could simply be achieved using 249 conventional vaccination with artificially lowered coverage. of (x, y A , y B ), many different combinations can lead to similar S n . For example, when antigenic change is 253 not limiting, we have already shown that when only manipulating y A (with x = 1) there are two values of 254 y A which ensure the optimal outcome, E[S final ] = 1 R. Therefore we have the opportunity to apply further 255 constraints to identify optimal combinations. One approach would be to apply additional metrics. For 256 example, in addition to maximizing S n we could also seek to maximize the average n at which S n is first 257 equal to 1 R (this was what separated y • from y * ), or minimize the variance between epidemic sequences 258 (when x = 1 2 and y A = y B ). Alternatively, we may impose the constraint that we are free to distribute 259 across targets (x) and then across both variants in the same way; that is, we take y A = y B = y and so we are 260 choosing the pair (x, y) rather than the triplet (x, y A , y B ). The motivation for imposing this constraint is 261 that doing so allows us to see when either distribution dimension has primacy. Specifically, when distributing 262 between targets is more important, then the optimal (x, y) will show variation between targets as coverage 263 (p and χ 0 ) changes, while all the doses will be allocated towards the primary variant, y = 1. In particular, 264 we expect to see optimal values x * and 1 − x * where x * ≠ 1 2, as observed when distributing across targets 265 only. When distributing between variants is more important, then we should expect y to vary with vaccine 266 coverage while doses will be evenly allocated across targets, x = 1 2.
Since for epidemics not limited by antigenic change the optimal vaccination strategy satisfies E[S final ] = 268 1 R, and as we have previously shown that this is always (theoretically) possible when we can vary doses 269 between variants, it logically follows that if we can vary doses between both targets and variants we can 270 similarly achieve the optimum. Therefore we will restrict our attention to when the epidemic is limited by Second, we assumed cross-protection was limited. The shape, or 'broadness' (i.e., how slow the decay 298 of χ z is for increasing z), of the cross-protection function is of no consequence in case I. In cases II and III, 299 however, the primary role of the broadness of vaccine cross-immunity is that it determines the reduction 300 in vaccine protection against new strains. If vaccine cross-immunity is broad, antigenic change will cause 301 a small reduction in vaccine protection, and so a smaller subsequent wave. If, however, cross-immunity is 302 narrow, antigenic change will cause a large reduction in vaccine protection, and so potentially a large wave.
Thus increasing cross-protection increases the efficacy of conventional vaccination in case II and III. For 304 example, if antigenic change is not limiting, conventional vaccination is more likely to achieve the optimum 305 of E[S final ] = 1 R with broad cross-immunity, whereas with narrow cross-immunity, substantial overshoot is 306 more likely (Fig. 2 ). In our model, the shape of cross-immunity will ultimately depend upon the units of 307 antigenic space. For example, if k i and k i+1 differ by a single, nearly-neutral mutation, cross-protection will 308 tend to be broad. If instead antigenic variation arises in a source population with pre-existing immunity, 309 large differences between k i and k i+1 may be necessary to escape extinction in the source population, and so 310 cross-protection may be narrow.
Third, we assumed each wave consists of a single strain. Although this may be reasonable if antigenic 312 variation originates elsewhere, it is less likely when variation is generated by de novo mutation. If our 313 results were extended to include multi-strain waves, the predicted efficacy of conventional vaccination would 314 be weakened, since whenever an escape mutant arises during an ongoing wave, there are more individuals 315 without infection-acquired immunity and so available to be infected by the escape mutant. This would lead 316 to a larger wave by the strain with limited vaccine protection. We have used a simple model to explore the optimal distribution of vaccines that differ in antigenic targets 319 and/or specific variants at those targets (i.e., strains). We show that with weak vaccines (either low coverage 320 or strength), evolutionary change in the pathogen population does not alter epidemiological outcomes and 321 so a mosaic vaccination strategy, in which individuals receive one of a set of possible vaccines, is not better 322 than a conventional, homogeneous strategy. In cases where pathogen evolution can lead to successive waves 323 of an epidemic, or is required for an initial wave, using a mosaic vaccination strategy often leads to better 324 outcomes than giving all individuals identical vaccines.
By assuming that vaccines designed to elicit immunity against different targets are equally effective, we 326 assume that distributing vaccine doses between those targets leads to no decline in initial protection relative 327 to conventional vaccines. The benefit of a mosaic strategy is that should there be any antigenic evolution 328 at either target that renders one vaccine ineffective (assuming limited cross-immunity between variants), 329 the other vaccine will still provide protection to the individuals that received it, and hence the population.
Perhaps surprisingly, the optimal distribution of vaccine doses between targets is not 50:50-better outcomes 331 can be achieved on average with unequal allocation, which will provide more protection in cases where the 332 target that is rarer among vaccine doses is the one that undergoes antigenic change.
It is interesting to consider our results in the context of the ongoing COVID-19 pandemic, given the We consider a pathogen with two vaccine targets, A and B, each of which can exhibit antigenic variation. 549 We let A i B j denote a pathogen strain with variant i at target A and variant j at target B. At each target, 550 antigenic space is one dimensional, such that as i (or j) increases, we move further away from the reference 551 strain, which we denote A 0 B 0 . Using this reference strain, we will refer to the variant indexed 0 as the dynamics are given by the system of ordinary differential equations
where i k is the variant of strain A B m at target k, i.e., i A = and i B = m, and so j − i k is the distance in 561 antigenic space between the variant targeted by the vaccine, j, and strain i variant, i k . From system (10), 562 the basic reproductive number of pathogen strain i is
where R ≡ βδ. It follows that if R 0 > 1, then an initially rare strain i will cause an epidemic.
The density of individuals who have not been infected by time t is 565
Our objective is to derive from system (10) an expression for the remaining uninfected individuals, S(t), 566 after sufficient time has elapsed for the epidemic wave to conclude, that is, we are interested in S(∞). To 567 do so, notice in system (10) that U is a monotonically decreasing function of time. Therefore, we can rescale 568 time by U , which will give the reduced system
From (13), we can directly solve for V kj , that is,
Using this result, we are left with the differential equation
As t → ∞, we know that I i → 0, that is, the wave of strain i will eventually burn itself out of the host 573 population. Therefore U (∞) is the solution of
which can be written, using Equation (14), as 
After the n th epidemic wave, the next pathogen strain, A B m , will have undergone n = + m antigenic 
indicates whether or not the (n + 1) th epidemic wave will occur. Note that, in general, U n 594 and V kj n will be specific to the sequence of antigenic changes.
If R (n+1) 0 ≤ 1, then U n+1 = U n , otherwise from equation (18), U n+1 is the solution of
satisfying 0 < U n+1 < U n . Then using Equation (14) we have
where i k is the variant at target k of the strain causing epidemic wave i, and
is the remaining uninfected individuals after the n th epidemic wave. We are therefore equipped to compute 599 the remaining uninfected individuals after the n th epidemic wave for a given sequence of antigenic changes.
Because the outcome, S n , will depend upon the sequence of antigenic changes, in the main text we focus denote the remaining uninfected individuals following a second epidemic wave by strain A j B k .
Then the expected remaining uninfected individuals following ω ∈ [0, 1] antigenic changes is
For epidemics not limited by antigenic change, the successive epidemic waves will continue until the 607 remaining uninfected individuals satisfies S final ≤ 1 R. In general this threshold will be reached within 1 or 608 2 antigenic changes (under the assumption of limited cross-protection, χ z ≈ 0 for z > 0). For epidemics not 609 limited by antigenic change, we restrict our attention to situations in which we either distribute vaccines 610 between targets, but not variants, (so vary x while fixing y k = 1) or between variants, but not targets (so 611 vary y A with x = 1). If we are distributing between targets, a single antigenic change is required to escape 612 vaccination at a given target; after this initial change, any subsequent antigenic change at that target will 613 have no effect. Therefore it suffices to consider two possible epidemic sequences, (i)
where S ( ) 3
is the remaining individuals without infection acquired immunity following the third epidemic 616 wave for epidemic sequence = i or = ii. If we are instead distributing between variants, since all vaccine 617 doses are directed at target A any antigenic change at target B will have no effect; moreover, it is possible 618 (depending upon y A ) that two antigenic changes at target A will be required to sufficiently deplete the 619 remaining uninfecteds. In combination, this means that S that is, we solve
for U 1 and χ 0 , then set χ 0 = TI II (p), which yields
The second threshold occurs when the epidemic wave by the initial strain A 0 B 0 is blocked, R obtain the threshold value of χ 0 for which this occurs (which we denote TII III (p)), we solve
optimal vaccination strategy, x * , is to distribute vaccine doses so as to maximize protection against one of 635 these sequences. Without loss of generality, focus upon the set of sequences in which the first antigenic 636 change occurs at target A, that is,
When we are considering epidemics limited by antigenic change, this means choosing x = x * to block the 638 second epidemic wave by A 1 B 0 , that is, x * satisfies R for U 1 , that is, we solve
for U 1 and then use that value in
to solve for x * . Solving the second expression for x * as a function of U 1 gives 642
while the first equation can be numerically solved for U 1 (there is no analytic solution of U 1 in this case).
Of course if x * is a solution then so to is 1 − x * .
If instead we are considering epidemics with unlimited antigenic change (and so limited by infection-645 acquired immunity), then we want to choose x * to satisfy S (A1B0) 2 = 1 R. Therefore we solve
for x * , U 1 , and U 2 . Note that if we are in case III, that is, χ 0 > TII III (p), then U 1 = U 0 = S 0 (1 − p), and we 647 only need to solve for x * and U 2 . 
that is, in case II, y A0 = 1, while in case III, since U 1 = U 0 , y A0 is the solution of
(2) 0 = 1, given the next strain is A 1 B 0 . That is, we solve
for y A1 and U 1 . Which of y A1 or y A0 performs the best will depend upon the values of ω and R.
Second, for epidemics not limited by antigenic change (and so limited by infection-acquired immunity), 658 we can always choose y A to ensure E[S total ] = 1 R. First, we can choose y A = y * such that after the first 659 epidemic S 1 = 1 R. To do so, we solve
for U 1 and y * ; doing so gives
.
Second, we can choose y A = y • such that S 2 = 1 R, if we assume the first antigenic change is A 1 B 0 (note 662 that this is justified since U 1 = U 2 = ⋯ = U j for A 0 B j , and so we can ignore all antigenic changes to target 663 B). Therefore, we solve
for U 1 , U 2 and y • . Finally, consider a vaccine which can be distributed across both targets and variants, but distribution across 667 variants must be the same across targets. That is, we are free to vary x and y A = y B = y. Here we focus 668 upon the situation in which antigenic change is limited, since we know from our previous results that when 669 antigenic change is not limiting, we can always reach the optimum E[S final ] = 1 R by varying between 670 variants alone.
The results are much as we would expect. When the probability of antigenic change is low (ω small),
it is best to distribute between targets while focusing upon the primary antigen, that is, choose y = 1 and 673
x = x * where x * is provided in equation (31) . As the likelihood of antigenic change increases, however, it 674 becomes optimal to distribute doses between variants, that is, y < 1, with doses equally allocated between 675 targets, x = 1 2. Specifically, if allocating between variants is the preferred strategy (and so x = 1 2), then 676 the optimal choice of y will block the next epidemic wave by either strain
is not sensitive to which strain emerges). Therefore we solve Figure 1 : Model schematics. Subplot a illustrates assumptions about pathogen structure (left) and genome (right; each line represents a unique pathogen genome, or 'strain'). We use 'targets' to describe different surface proteins (or, equally, different epitopes on the same surface protein). We use 'variants' to capture genetically (and more crucially, antigenically) distinct versions of a given antigenic target. Subplot b gives the density of individuals vaccinated against different pathogen strains. A fraction p of all individuals in a population, S 0 , receive a vaccine. Vaccine doses are distributed across targets (a fraction, x, focus on target A, while 1 − x focus on target B) and variants (a fraction, y A or y B , focus on the primary variant of the given target). Subplot c illustrates the typical dynamics of our model, where S n denotes the density of susceptible individuals (red curve) remaining after the n th wave of the epidemic. The blue curve captures our assumption that each wave reaches its epidemiological conclusion before another can begin, if the density of susceptibles is above 1 R. One possible epidemic sequence of strains is shown in green. Figure 2 : The three qualitatively different epidemic regimes under conventional vaccination. In case I, a single epidemic by the initial strain A 0 B 0 occurs. In case II, there can be two epidemics, one by the initial strain, and one by the strain A i B j (here assumed to be A 1 B 0 ). In case III, the initial strain is blocked and a single epidemic wave occurs by a secondary (antigenically novel) strain (again, assumed here to be A 1 B 0 ).
Here we show the outcome in terms of the remaining uninfected individuals, S n , as we vary vaccine coverage, p, and vaccine strength, χ 0 , where the upper translucent surface is what occurs with no antigenic change, whereas the lower surface is what occurs when antigenic change is a certainty. The curves delineating the thresholds between cases, TI II (p) and TII III (p), are shown for reference, while the insets show examples of the epidemic dynamics for each case. Figure 3 : Optimal distribution of vaccines between targets when antigenic change is either limiting (with probability ω = 0.9, subplots a,c) or is not limiting (subplots b,d). Subplots a,b show the optimal distribution as vaccine coverage, p, and strength, χ 0 , vary. Note that when x * is optimal (blue surface), 1 − x * is also optimal (red surface). The thresholds TI II (p) and TII III (p) are also included for reference; beyond the green surface, vaccine efficacy is sufficiently low that evolution has no effect on the epidemic, and any distribution of vaccines between targets will produce the same outcome (for a given p and χ 0 ). Subplots c and d reveal the logic. Consider epidemics limited by antigenic change (subplot c). When x is small, most vaccine doses are distributed against target B. Consequently, antigenic change at target A can only slightly reduce vaccine protection, and so the potential epidemic wave by strain A 1 B 0 will be blocked (solid blue line). As x increases to x * , the potential epidemic wave by A 1 B 0 remains blocked, while protection against strain A 0 B 1 increases, reducing the size of the potential epidemic wave by A 0 B 1 (dotted blue line). At x * , R
(2) 0 = 1 for strain A 1 B 0 , and so although any further increases to x will increase protection against A 0 B 1 , it will do so by reducing protection against strain A 1 B 0 . Therefore, x * is optimal, i.e., it maximizes the density of individuals who remain uninfected on average (E[S final ], red line). A similar argument holds if we focus upon the sequence A 0 B 0 → A 0 B 1 and the optimal strategy 1 − x * . Next, consider epidemics not limited by antigenic change (subplot d); as before, focus upon the sequence A 0 B 0 → A 1 B 0 → ⋯. When x is small, although the epidemic wave by strain A 1 B 0 will be blocked, eventually there will be antigenic change at target B. This new strain, say A j B 1 with j ≥ 1, will have escaped all vaccine protection (since antigenic change is cumulative and cross-protection is limited), and so will cause a large epidemic and substantial overshoot. As x increases, eventually R
(2) 0 > 1 for strain A 1 B 0 , that is, strain A 1 B 0 will cause a (small) epidemic wave. In turn, this will deplete the available susceptibles and so reduce the size of any subsequent epidemic wave by strain A j B 1 , limiting overshoot. When x = x * , vaccine protection against strain A 1 B 0 is sufficiently weakened such that following an epidemic wave by strain A 1 B 0 , S final = 1 R. Any further increase to x will reduce protection against A 1 B 0 such that it causes overshoot. Similar logic follows for the sequence A 0 B 0 → A 0 B 1 → ⋯ and the optimal solution 1 − x * . In subplot a, y A0 and y A1 represent the optimal strategy protecting against either the primary (y A0 ) or secondary (y A1 ) variant at target A; as the likelihood of antigenic change increases, then the likelihood y A1 is the optimal strategy increases. In subplot b, the red and blue surface represent the two optimal strategies, y * and y • when the epidemic is limited by infection-acquired immunity. Both satisfy E[S final ] = 1 R, however y * satisfies S 1 = 1 R, while y • requires at least one antigen change, which would result in more than one epidemic peak and overall slower spread in the host population. As in Figure  3 , beyond the green surface, vaccine efficacy is sufficiently low that evolution has no effect on the epidemic outcome, so the best vaccination strategy is to focus on the primary variant. In both panels, the red surface corresponds to the optimal distribution when antigenic change is unlikely (i.e., ω is small) while the blue surface corresponds to the optimal distribution when antigenic change is likely (i.e., ω approaches 1). As before, beyond the green surface, vaccine efficacy is sufficiently low that evolution has no effect on the epidemic outcome. In general, when antigenic change is unlikely (red surfaces) we should distribute between targets and protect against primary variant (y = 1), whereas when antigenic change is likely (blue surfaces) we should distribute between variants with doses evenly allocated between targets (x = 1 2). Note that for visual clarity in panel a, we have only shown one possible optimal solution; but if x is optimal, then so is 1 − x. See Sup. Info. 5.6 for full derivation.