Immune memory and vaccination

The most useful feature of adaptive immunity is that it remembers. All of vaccination rests on this.

Primary and secondary response

   antibody
   quantity
        |                                        ___
        |                                      _/   \__      SECONDARY
        |                                    _/         \_   response
        |        ___                       _/
        |      _/   \___                 _/
        |    _/         \___          __/
        +---|---------------|--------|-------------------> time
          1st contact              2nd contact
          (7-10 days)              (2-3 days)

                   PRIMARY RESPONSE      SECONDARY RESPONSE
delay              7 to 10 days          2 to 3 days
intensity          moderate              100 to 1000 times stronger
duration           short                 prolonged

The difference lies in memory cells: lymphocytes selected at first contact and sometimes kept for life. At the second contact, clonal selection no longer has to be redone — the army is already raised.

This is why measles or chickenpox are caught only once: the second contact is neutralised before it can even cause symptoms.

The principle of a vaccine

A vaccine triggers a primary response without the disease:

   the ANTIGEN alone is introduced, without its pathogenic power
              |
              v
   primary response  ->  MEMORY cells
              |
              v
   on real infection: immediate secondary response

   inactivated or attenuated virus  flu, measles, polio
   fragment of the agent            hepatitis B, papillomavirus
   messenger RNA coding the antigen Covid-19

Historically, Jenner observed in 1796 that milkmaids who had caught cowpox — a mild disease — did not develop smallpox. Pasteur generalised the principle a century later and named it after the cow (vacca).

The outcome is the only complete eradication of a human disease: smallpox, declared eliminated in 1980 after killing some 300 million people in the 20th century.

Herd immunity

A vaccinated person protects themselves, but also others: they stop being a link in transmission.

   herd immunity threshold  =  1 - 1/R₀

where R₀ is the average number of people infected by one case in an unprotected population.

disease      R₀        threshold
--------     -------   ----------
flu          ~1.5      33 %
Covid-19     ~3        67 %
measles      ~15       93 %

Measles therefore requires 93 % coverage. This is why it returns as soon as vaccination slips by a few points: the threshold is so high that it tolerates almost no margin.

This indirect protection is what shields those who cannot be vaccinated: infants, the immunocompromised, patients on chemotherapy.

What a vaccine does not do

- it does not protect INSTANTLY: 1 to 2 weeks are needed
- it does not always protect FOR LIFE: some need boosters
- it is not 100 % effective: but an infection in a vaccinated person
  is generally far less severe

These limits are real and documented; they do not undermine the benefit, which is measured at population scale.

Summary

• The secondary response is far faster and stronger than the primary one.
• It rests on memory cells, sometimes kept for life.
• A vaccine triggers a primary response without the disease.
• Herd immunity protects the unvaccinated too; threshold 1 - 1/R₀.
• Measles: R₀ ≈ 15, hence 93 % coverage needed.
• Smallpox is the only eradicated human disease (1980).