Mishna Berura - Effectiveness
The Ba’er Heteiv (Orach Chaim 334:22), and Mishna Berura (334:22) rule that one saves a healthy person from a fire before a dangerously ill individual. In other words, the one for whom one’s efforts will be efficacious, enjoys priority. The source of this ruling is the Sefer Chassidim number 424 (in the Margolios edition). However, the Sefer Chassidim does not provide a source in the Gemara for his ruling. The source is also apparent from the selections from the Gemara cited above. As TABC Talmid Nachi Scheiner notes, the Gemara in Kiddushin 29b writes that a family’s resources should be allocated to help the family member whose learning will be most effective.
The Ba’eir Heitev and Mishna Berura’s approach is followed in practice by the Poskim of the current and past generations. These Poskim include Rav Moshe Feinstein (Teshuvos Igros Moshe Choshen Mishpat 2:73:2), Rav Shlomo Zalman Auerbach (Teshuvos Minchas Shlomo 2:86), Rav Eliezer Waldenburg (Teshuvos Tzitz Eliezer 17:72), and Rav Asher Weiss (Teshuvos Minchas Asher 1:115 and 2:126).
TABC Talmid Zachary Becker notes that based on the Ba’eir Heitev and Mishna Berura, one who has not contracted Covid takes priority over someone who is severely ill with Covid. TABC Talmid Boaz Kapintanker argues, though, that the thing is, that there is a direct return to the party giving the resource. If we were to apply the same thing to our case, and say that the government should give the vaccine to the healthiest people, the people aren't providing direct benefit back to the government. It's a different relationship between a family-student versus a government-citizen.
Accordingly, Poskim accept both the criterion of the Pri Megadim (greater need) and the criterion of the Mishna Berura (greater effectiveness). While Rav Shlomo Zalman and Rav Weiss cite both criteria and do not see them as competing, Rav Gedalia Schwartz (Shaarei Gedula page 355) seems the Pri Megadim and Rav Moshe as disagreeing.
While there are cases when these criteria do not compete, there are times that they do. It is not clear, however, what should be done in case where these two criteria compete with each other. In such a case it is not clear from the Poskim which enjoys priority.
3.
Rav Yaakov Emden (Migdal Oz Even Bochein 1:92) rules that a young person is saved before an older person. TABC Talmi Jacob Becker argues that this refers to a very large discrepancy in ages. This might accord with the Mishna Berura’s standard of the more effective action enjoys priority. Nonetheless, Rav Moshe Feinstein (Teshuvos Igros Moshe 2:75:7) writes that it is “Mistaber” (reasonable) that one does not grant priority to a younger person. Rav Shlomo Zalman Auerbach (ad. loc.) resolutely rejects the possibility of age serving as a factor. Finally, Rav Asher Weiss (Teshuvos Minchas Asher 2:126) is unsure of this issue. He writes that he is unaware of any source upon which one may basis an answer to this question.
Rav Schachter (Piskei Corona #15) notes that we do not choose a younger person over an elderly person just because the former is expected to live many more years. The reason is that the elderly person may contribute more to society in the five years he will live than the younger person will in the forty years he will live. This idea runs along the lines of the Mai Chazis principle we cited earlier.
However, Rav Moshe (Teshuvos Igros Moshe 2:73:2) does concede that we choose a terminally ill person expected to live only a brief time - “Chayei Sha’ah” - over a healthy individual expected to live a normal lifespan. Indeed, we find the Gemara (Avoda Zara 27b) where the Gemara teaches that we choose the chance to achieve a full lifespan over “Chayei Sha’ah” (L’Chayei Sha’ah Lo Chaishinan). Of course, as Tosafos (Avoda Zara 27b d”h L’Chayei Sha’ah) explain, we respect and are concerned for Chayei Sha’ah. However, when faced with a choice between a normal lifespan and Chayei Sha’ah we choose the opportunity to achieve a typical lifespan.
Rav Moshe (Teshuvos Igros Moshe 2:75:2) in this context defines Chayei Sha’ah as one year. Rav Asher Weiss (Teshuvos Minchas Asher 1:115), however, argues that Chayei Sha’ah applies only to one who is expected to live six months or less. Rav Weiss bases his ruling on medical studies that indicate that while physicians are able to accurately assess a life expectancy of six months or less, many predictions of longer length of life are proven inaccurate.
TABC Talmid Yaakov Halstuch suggests that Gemara Sanhedrin 32b can be marshaled as evidence for Rav Yaakov Emden according priority to the younger person. The Gemara grants priority to the boat with a longer distance to its destination which may parallel the younger individual, who has a longer life expectancy than the older individual.
TABC’s Tzvi Meister, though, notes that in consideration of the existing medical data, we may be more likely to rule out Rav Yaakov Emden and the subsequent rulings as not applicable, or at least only applicable in Rav Asher Weiss' ruling, as data shows that SARS-Cov-2, which causes Covid-19, is mostly only deadly to older individuals. Rav Emden's blanket conclusion is not easily applicable here given the issues of who carries the most risk or certainty of being harmed from catching.
In this case the Pri Megadim’s criterion of greater need conflicts with the Mishna Berura’s criterion of greater benefit.
Finally, we should note that typically the Halacha accords priority to the older individual. For example, in signing a Shtar common practice is to grant the older person the earlier signing position. This practice stems from the Gemara (Gittin10b) “Ravcha Shavik LeMan DeKashish Mineih”.