Found it

I've been trying to find this quote again for a while:

In short, man's modern skill in the prevention and control of disease has succeeded to some extent in stripping civilized populations of their natural immunities, leaving them acutely vulnerable when virulent germs do attack.  It is difficult to foresee a solution to the problem.  The microbes that can cause disease are too prolific, too adaptable and too widespread to be made to disappear from the face of the earth overnight.

Nor can vaccines supply the whole answer.  So many kinds of bacteria and viruses exist that at present it is impossible to vaccinate people against all of them.  The great success of the smallpox vaccine comes from the fact that until now only one strain of smallpox virus has been known.  The pneumococcus and streptococcus bacteria, on the other hand, together have at least 100 different strains; a vaccine against only one of them would be next to useless.

To complicate matters further, some germs have an alarming way of developing new strains that are unaffected by existing vaccines and drugs....

Useful as modern drugs may be in treating or curing disease, they are now undergoing a critical reappraisal as it becomes clear that microbes have powers that remain beyond the magic of medicine.  Sanitation, rising living standards and vaccinations, without the aid of drugs, have brought the major epidemics under control in most of the industrialized nations.  Thanks to drugs, man's burden of illness is greatly eased.  But despite recent optimism, microbial diseases are a constant threat.

It's from the Time-Life Life Science Library volume Health and Disease, from 1980. Forty years ago, and the situation now is not much different.

From here, another interesting quote:

On average, the annual flu vaccine is around 40 percent effective.

Compare that to about 90% effectiveness for other vaccines, because different flu strains are prevalent every year, and the ones they put in the vaccine are only a guess.

I've contended before that the strains in the vaccine will almost never match the strains in widest circulation, because mass vaccination leaves the "potential sickness space" to be occupied by other strains. It is worth quoting:

Somewhere I once read a statement by a doctor (I wish I had the reference, but I don't) saying that vaccination could only ever be an effective strategy against a limited number of slowly-mutating micro-organisms.  Limited, because there is a physical limit to how many vaccinations a human body can receive and produce immunity from. Slowly-mutating, because the antibodies that are created to identify and destroy a particular threat won't be effective if it has changed so much that they can't recognize it.

Influenza is not slowly-mutating, and has a variety of strains. So, for the flu shot, each year "they" try to guess what the most prevalent strains will be (say, strains A, B, and C), and the manufacturers produce a vaccine that targets these strains. A fairly high percentage of the population dutifully get their flu shots, and for most of them it is effective: they don't fall ill with strain A, strain B, or strain C, and so those strains don't spread through the population. Instead, the flu strains that go around are strains D, E, and F. Health officials issue a statement:  "Oops, we guessed wrong. We promise to do better next year, so be sure to get your flu shots then!" Rinse, and repeat the whole sequence the following year.

So, the strains that are in your flu shot are never the strains that are actually in circulation around you that year. Probably widespread flu shots do help somewhat to reduce the total number of flu cases, by blocking the spread of the frontrunner strains each flu season. But they can't be counted on to protect any particular person from getting the flu, and it is bad science to claim that they can. (Public health policy operates on the assumption that People Are Idiots, and shamelessly uses bad science and propaganda to promote its various initiatives. But actually, only Most People Are Idiots.)

Supposedly, this year's vaccine is relatively effective, combating the strains that were going around last year, but somehow at the same time 2019-2020 has been a relatively bad flu season.