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Conjugate vaccines
  1. A Finn1,
  2. P Heath2
  1. 1Institute of Child Life and Health, Clinical Sciences at South Bristol, University of Bristol, UK
  2. 2Dept of Child Health, and Vaccine Institute, St George’s Hospital Medical School London, UK
  1. Correspondence to:
    Prof. A Finn
    David Baum Professor of Paediatrics, University of Bristol, Institute of Child Life & Health, Dept Clinical Sciences at South Bristol, Level 6, UBHT Education Centre, Upper Maudlin St, Bristol BS2 8AE, UK; Adam.Finnbristol.ac.uk

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Time for more of them or less of them?

It all used to seem so simple with conjugate vaccines. You added them to your infant schedule and, faster than anyone had dared to hope, the disease more or less vanished.1,2 Not only did immunisation protect against invasive disease but it reduced upper respiratory carriage rates too,3 so there was herd immunity. Even when odd, unexpected mixing problems cropped up out of the blue—like acellular pertussis and Haemophilus influenzae type b (Hib) combinations4,5—it didn’t really seem to matter.6 In 1999 we watched as meningococcus group C (MenC) set off down the path to oblivion7,8 previously trod by Hib in 1992.9

Then, suddenly, with the arrival of the new millennium, it began to get more complicated. With the MMR vaccine scare still buzzing in people’s heads and the schedule busier with the addition of MenC, the 7-valent pneumococcal conjugate vaccine turned up in 2001 with a central European licence and unassailable evidence showing that it prevents invasive pneumococcal disease10 leading to its general introduction in the USA in 2000. The way this vaccine arrived was in stark contrast to MenC—which was a programme driven by strategic thinking from within the UK Dept of Health11 in which three manufacturer’s had responded to the call and an enthusiastic public waited impatiently for a partial solution to the problem of meningococcal disease which they knew of and dreaded. Here was a vaccine specifically developed by one manufacturer, who had, by a combination of luck and shrewd judgement, stolen the march on all other contenders, with the US market as their primary target. (The luck was that the conjugates in their vaccine turned out to be very immunogenic: the US Food and Drug Administration takes the very inflexible view that new vaccines admitted to the marketplace must be at least as “good”, as judged by immunogenicity, as any already there. As a result, development of several subsequent vaccines by other manufacturers, all of which would almost certainly have been highly effective,12 has been dropped as it became apparent they would not reach the lucrative US market). Anyway, the British public had never heard of pneumococcus—the name suggested pneumonia not meningitis, and people thought pneumonia was an illness of the elderly. So a new vaccine was suddenly available, which few people, if any, realised they wanted or needed and the purchase of which would cost more, by some distance, than all the existing routine childhood vaccines put together. On the face of it, this was hardly a five star political opportunity.

Just as the prolonged period of head scratching about this got underway, it was noticed that more cases of Hib disease were occurring.13 For a decade the UK had been using three priming doses of Hib vaccine in early infancy and, unlike most anyone else, no boosting dose in the second year, and apparently getting away with it.14,15 But not any more. The lower primary antibody responses to Hib when combined with acellular pertussis vaccines first noted in the mid-1990s4,5 turned out to matter after all.16 In addition, the cohort of older children immunised in the first catch up programme in 1992–93 had now grown up so that all children aged up to 10 years had received Hib vaccine only in infancy. The catch-up campaign’s contribution to herd immunity had been more important than previously appreciated. In 2003 a repeat catch-up programme was implemented to plug the gap.17 But seeds of doubt concerning unboosted priming schedules had been sown. Sure enough, data on MenC effectiveness has duly confirmed that long term protection of those immunised with three doses as infants is poor, but is much better in those immunised with only one dose aged 1 year or older.18

So policy makers are now confronted not only with the question of whether and how to introduce universal conjugate pneumococcal immunisation19 but also questions about the best future schedules for Hib and MenC. It would make sense to make changes to the schedules for these three vaccines all at once rather than piecemeal, given the need to keep everyone involved properly and clearly informed and considering that even the introduction of simpler and less reactogenic vaccines in September 2004 induced a flurry of anxious media reports.20 A quirk of history (the decision to undertake initial Hib conjugate immunogenicity studies in Finland using their three dose DTP schedule21) determined the design of initial efficacy studies22,23 and precipitated the common licensure and use of three priming Hib vaccine doses in infancy. This in turn seems to have set the standard for subsequent thinking about all conjugate vaccines, even though the Finns themselves went on to show that two priming doses were fine24 and use a schedule of two doses in early infancy and a booster early in the second year to this day.25 They even showed good priming, despite somewhat lower antibody responses, after only one dose of Hib–OMP and –tetanus conjugate vaccines,26 the immunological basis for which has been the subject of subsequent study.27 Phase two studies with MenC CRM197 conjugates also suggested that the majority of antibody is generated after two doses28 and the decision to use three priming doses in the UK may have had more to do with “keeping things simple” (see below) than any real need for that many doses. The MenC tetanus conjugate appears to be highly immunogenic after two or even just one priming dose29 and fewer priming doses also appear to induce larger memory responses to subsequent boosters.29 A recent UK study commissioned by the Department of Health to explore alternative regimens for the conjugate pneumococcal vaccine likewise suggests that two priming doses may be enough,30 and a recent study from the Philippines showed similar antibody concentrations at age 9 months, after one or three doses of an 11-valent pneumococcal diphtheria/tetanus conjugated vaccine given in infancy.31

The Americans seem to take a different approach towards the design and logistics of their infant and early childhood schedule. A new vaccine is shown to be safe and efficacious, the cost-benefit argument is rehearsed, and the change is made. More injections, more visits, that’s just the way it is. In the USA, by the age of 5 a fully immunised child has received up to 24 injections (not including flu) given over about seven visits. Over this side of the Atlantic the issues of what is acceptable to and practical for parents (we can easily guess what the children might say, if asked!) and the primary care staff delivering the vaccines seems to have more influence on decisions. Giving exactly the same vaccines at all infant visits is a lot simpler and less error prone than having different ones each time and has been the rule for the first three UK immunisation visits to date. Giving a smaller number of injections or a single combination vaccine, rather than multiple injections at any single visit is much less distressing to all concerned and the latter approach becomes downright impossible with some pre-school children, necessitating additional appointments. Add to this the observation that not only can combining conjugate vaccines in the same syringe result in changes in their immunogenicity32 but so can giving them at different sites at the same time,33 and deciding how to deliver, say, two priming doses each of Hib, MenC, and pneumo conjugates alongside three doses each of DTP and polio vaccines—ignoring, for the moment, hepatitis B—becomes quite a puzzle. Literally dozens of possible options are conceivable and, so far, no other country has really pointed the way.

But the European Union is bigger now than it was before and includes countries that are not yet systematically using Hib vaccine.34 This is at least in part because of the purchase costs of the vaccines.34 That being the case, general European use of conjugate pneumococcal vaccine, which is much more expensive than Hib vaccines, by the schedule used in the USA and under which it is licensed in Europe (three doses in infancy followed by one in the second year of life) or even a “reduced” two dose priming course followed by booster, seems a long way off. Indeed, since manufactured supplies of pneumococcal conjugate vaccine have at times been insufficient to meet demand in the USA, it seems more likely to be logistically feasible to provide fewer than four doses to every European child, if this proves to be acceptable and effective. The latest data from the USA, now four years into general use of 7-valent conjugate vaccine, in addition to continuing to show impressive overall effectiveness in vaccine recipients, also shows remarkable levels of herd immunity among unimmunised age groups.35,36 Imagine that a country decided it wished to reduce the burden of invasive pneumococcal disease but that it could afford or obtain only one dose of the conjugate vaccine per child: it might choose to administer it at around a year of age, at a time when a single dose might reasonably be expected to induce substantial antibody responses and immunological memory and reasonable long term protection in recipients. This is certainly now the experience with single doses of Hib and MenC administered as one dose only in older children.18,37 Indeed some countries (for example, the Netherlands and Australia) newly introducing MenC have adopted this one dose schedule. Of course this approach would deliver vaccine too late for many, if disease patterns in infants remained the same. But what if significant herd immunity effects were observed? In the case of pneumococcal disease perhaps immunised toddlers would introduce fewer vaccine-type strains into their households38 so that invasive disease rates would drop in their infant siblings too?

Given slightly larger but still limited budgets, if money were saved by withholding infant doses and was instead used to purchase vaccine to immunise older children in a catch-up programme at the outset, would the resulting additional extra public health benefits of broader paediatric herd immunity outweigh those of more conventional infant immunisation? Mathematical modelling may help address this. A programme of this kind in a single small European country with an efficient delivery machinery for paediatric vaccines and a well organised clinical and microbiological surveillance system to pick up cases of invasive disease could not fail to deliver significant public health benefit locally and might teach us much about the potential power of herd immunity in the use of pneumococcal conjugate vaccines.

As we look back at the first decade and a half of use of conjugate vaccines in young children, in our excitement at the discovery of a tool that works in infants who are the most common victims of invasive community acquired bacterial infections, perhaps we have paid too little attention to the capacity of these vaccines to induce herd immunity. Instead of seeing this simply as an added bonus, perhaps we should put it at the centre of future strategic planning. Although fears about “immunological overload” are unfounded, no one would argue that today’s childhood vaccine schedules are busy and set to get busier. Furthermore, many licensed vaccines of known efficacy are not yet in general use in many places, at least in part because of competing priorities on the public purse. Examples in the UK include hepatitis B and varicella vaccines and broader use of flu vaccine in healthy children, as well as the conjugate pneumococcal vaccine discussed above. In this setting, an international collaborative approach to vaccine studies designed to optimise and rationalise (and, who knows, maybe even “harmonise”) current schedules, which are based as much on tradition as science, may be timely. Even if individual countries remain too stubborn to consider adopting “foreign practices”, it certainly appears that, in some cases, the numbers of doses of conjugate vaccines used might safely be reduced.

Time for more of them or less of them?

REFERENCES

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Footnotes

  • Competing interests: The University of Bristol and St George’s Hospital Medical School have received funding for research projects and consultancy on behalf of Adam Finn and Paul Heath respectively from Wyeth who manufacture 7-valent pneumococcal vaccine and also from other manufacturers of conjugate vaccines

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