该问题已经被湖南师范大学易仲良教授利用“必须省或强制省”和“任选省”解决，该文章刊登在西安外国语大学主办的《外语教学》1999年第一期上，有兴趣的可以从https://mall.cnki.net/magazine/Article/TEAC901.015.htm下载。

This result is better than ___ would probably be achieved by a vaccination policy.

这一句的主语省略了，本不该省略。只能说这样的例子很特别，也很少见。一般大家都能看出省略了the result，但是正如王老师所说，你要把它补上去是不合适的，因为重复是很忌讳的，除非不得已。所以可以写成：

This result is better than the one that would probably be achieved by a vaccination policy.

写成what形式的当然也可以：

This result is better than what would probably be achieved by a vaccination policy.

为了说明是怎么回事，要找类似than后接被动语态的例句，这太费时间，还是找到一个：

In their paper, Dr.Mansfield and Dr. Wildberger show that this is better than what would be calculated using a trigonometric table from the Indian mathematician Madhava 3,000 years later.（纽约时报20170830）

接下来说说我所找到的外刊（限于纽约时报，经济学人和卫报）than后省略主语（被动）的情况，并说明：

1. I undertook of my own accord a much greater task than had been imposed.

这是常见的形式，比较要素充当了than后的主语，当然要强制省略。

2. The process offers the prospect of cleaner emissions at lower cost than can be achieved with the methods now used.

3. The innovation of Quirk was to build a corpus of continuous texts and spoken transcriptions, selected so as to capture a wide range of contemporary text types and spoken genres – thereby representing the use of the language in terms of time and variety more strictly and consistently than had been achieved in earlier periods.

4. Spain’s prime minister, Jose Maria Aznar, though still insisting that the fight against ETA comes first, has responded in a far mellower tone than had been heard for many months.

5. For example, nanoscale manufacturing allows the design of probes that can monitor biological experiments with far greater sensitivity than can be achieved today.（经济学人20011222）

2-5句省略than后的主语是因为本来比较要素是一个副词短语，但是因为向上渗透的原因，整个比较要素的范围扩大了，而这个扩大的比较要素充当了than分句的主语，因此全部省略了。

6. Farmers’ organisations argue that the country should not import foods that are harder on the environment or on animals than would be allowed under domestic rules.

这一句的特别之处在于foods后接了个定语从句，会向上渗透至整个foods that are harder on the environment or on animals短语。而这个短语又是than分句的主语，故全部省略。

王老师举的被动的例子里面有一些就是上述的类似情况，剑桥语法还是能解释。只有以下的7-10句跟开头的那个例句很像（我注明了出处），没法说。

7. Winch's study is considerably more detailed than has been described. （Consistency in Cognitive Social Behaviour: An introduction to social psychology一书的第103页）

8. This frequency of non-albicans infections is higher than has been described historically. （Infectious Diseases一书第2200页）

9. Control system performance must be noticeably better than was achieved by the PI controller shown in Figure 7.80. （没找到，可能是一本专业杂志的）

10. As has been pointed out, these gains “per hour” are not necessarily better than has been achieved by some other studies over a longer period.（Dyslexia in Context: Research, Policy and Practice一书的第125页）

反过来，这也说明剑桥语法举的上面的那个例子不是孤例，尽管很少，二十多年前人家就预料到了。我在近些年的纽约时报，经济学人和卫报上找过，没有发现这样的，找到的都是能解释的。也可能是我找的不够细。不管怎么说，这种情况下不省略应该是规范的英语。遇到这种省略的情况也不必奇怪，语言是多变的，也是演化的。

我的观点在这个链接中已经表明了。注意文章最后我引用的Fowler在《现代英语用法词典》中的一段话，就是对这种句子的一种批评。比较分句than what中的what应该保留还是删去 - 柯帕斯英语网 (cpsenglish.com)

我理解 result 应该是比较对应项，比较的是 result. 这句话的意思是：此结果比接种疫苗政策带来的结果更好。该书下面也写道：比较分句相当于 than the result that would probably be achieved by a vaccination policy. 这种省略是常见的。如：More is written than is read. (外媒例句）。当分句比较项为被动语态主语时，现代英语，特别是美语中这种省略很普遍。

如果不省略，就需要用 what, 即 than what would probably achieved by... 这样也是对的。

请看例句：

1. All of these results are better than what was achieved by rapid type analysis.

2. The resultant fibre is qualitatively better than what has been achieved through conventional retting. 

实际上，如果原句写为 than the result that would... 反而不是好的文体，因为英语忌讳重复。我理解该书作者是考虑为读者讲解的需要在下面予以补全分析，但这并不意味着英语必须这么说。很多网友喜欢所谓“补全”，其实很多句子补全后是多余的，文体显得不简练。

但该书作者在下面还提到以上例句是否符合语法是令人质疑的，但作者并没有否认这种用法的正确性，因为英语中确实有这样的用法。该部分排版时设计为蓝色阴影段落，作者指出这种段落不是针对普通读者的，即使忽略也不影响理解。请看该书的解释：

我理解，该书几个例句的用法，多见于美国英语，也许因为作者是英国人，所以认为上面的用法令人质疑。实际上，在美国书刊中这种用法很常见，我找了几个和书中例句结构类似的外刊例句，请比较：

1. Control system performance must be noticeably better than was achieved by the PI controller shown in Figure 7.80. 

2. As has been pointed out, these gains “per hour” are not necessarily better than has been achieved by some other studies over a longer period.

3.He resold space to advertisers in one-inch units at prices much higher than he paid. 

4.He was in need of men and although the price was high, considerably higher than he paid his own men, he had little choice.

5. The distributor sells products at a mark-up price higher than what he paid to the manufacturer.

6. The log houses are better than they had at CampLincoln.

7. I suppose their living conditions are better than they had in Russia.

8. But its size is small, much less than actually occurred.

9. In 1960 and 1965, we forecast fall flights less than actually occurred.

以上仅是本人的理解，如有不准确的，欢迎大家指正。