Editing Herd immunity (section)

==Boosts==
{{Further|Artificial induction of immunity}}

===Vaccination===
{{Main|Vaccination|Vaccine}}
{{See also|Vaccine-preventable disease|List of human disease case fatality rates}}

The primary way to boost levels of immunity in a population is through vaccination.<ref name=pmid21427399/><ref name=pmid22561998>{{cite journal|vauthors=Rashid H, Khandaker G, Booy R|title=Vaccination and herd immunity: what more do we know?|journal=Current Opinion in Infectious Diseases|volume=25|issue=3|pages=243–9|date=June 2012|pmid=22561998|doi=10.1097/QCO.0b013e328352f727|s2cid=19197608}}</ref> Vaccination is originally based on the observation that milkmaids exposed to [[cowpox]] were immune to smallpox, so the practice of [[inoculation|inoculating]] people with the cowpox virus began as a way to prevent smallpox.<ref name=pmid23760373/> Well-developed vaccines provide protection in a far safer way than natural infections, as vaccines generally do not cause the diseases they protect against and severe adverse effects are significantly less common than complications from natural infections.<ref>{{cite journal|vauthors=Maglione MA, Das L, Raaen L, Smith A, Chari R, Newberry S, Shanman R, Perry T, Goetz MB, Gidengil C|display-authors=6|title=Safety of vaccines used for routine immunization of U.S. children: a systematic review|journal=Pediatrics|volume=134|issue=2|pages=325–37|date=August 2014|pmid=25086160|doi=10.1542/peds.2014-1079|url=http://www.escholarship.org/uc/item/2f93s53t|doi-access=free|access-date=5 July 2019|archive-date=30 January 2020|archive-url=https://web.archive.org/web/20200130171937/https://escholarship.org/uc/item/2f93s53t|url-status=live}}</ref><ref>{{Cite journal |last1=Di Pietrantonj |first1=Carlo |last2=Rivetti |first2=Alessandro |last3=Marchione |first3=Pasquale |last4=Debalini |first4=Maria Grazia |last5=Demicheli |first5=Vittorio |date=2021-11-22 |title=Vaccines for measles, mumps, rubella, and varicella in children |journal=The Cochrane Database of Systematic Reviews |volume=2021 |issue=11 |pages=CD004407 |doi=10.1002/14651858.CD004407.pub5 |issn=1469-493X |pmc=8607336 |pmid=34806766}}</ref>

The [[immune system]] does not distinguish between natural infections and vaccines, forming an [[Immunity (medical)#Active immunity|active response]] to both, so immunity induced via vaccination is similar to what would have occurred from contracting and recovering from the disease.<ref name=pville>{{cite book|vauthors=Pommerville JC|year=2014|title=Fundamentals of Microbiology: Body Systems Edition|url=https://books.google.com/books?id=Alz3BQAAQBAJ&pg=PA559|publisher=Jones & Bartlett Publishers|pages=559–63|isbn=978-1284057102|access-date=30 March 2015|archive-date=1 May 2021|archive-url=https://web.archive.org/web/20210501053032/https://books.google.com/books?id=Alz3BQAAQBAJ&pg=PA559|url-status=live}}</ref> To achieve herd immunity through vaccination, vaccine manufacturers aim to produce vaccines with low failure rates, and [[vaccine policy|policy makers aim to encourage their use]].<ref name=pmid22561998/> After the successful introduction and widespread use of a vaccine, sharp declines in the incidence of diseases it protects against can be observed, which decreases the number of hospitalizations and deaths caused by such diseases.<ref>{{cite journal|vauthors=Papaloukas O, Giannouli G, Papaevangelou V|title=Successes and challenges in varicella vaccine|journal=Therapeutic Advances in Vaccines|volume=2|issue=2|pages=39–55|date=March 2014|pmid=24757524|pmc=3991154|doi=10.1177/2051013613515621}}</ref><ref>{{cite journal|vauthors=Shann F|title=Nonspecific effects of vaccines and the reduction of mortality in children|journal=Clinical Therapeutics|volume=35|issue=2|pages=109–14|date=February 2013|pmid=23375475|doi=10.1016/j.clinthera.2013.01.007}}</ref><ref>{{cite journal|vauthors=Visser A, Hoosen A|title=Haemophilus influenzae type b conjugate vaccines - a South African perspective|journal=Vaccine|volume=30|issue=Suppl 3|pages=C52-7|date=September 2012|pmid=22939022|doi=10.1016/j.vaccine.2012.06.022|hdl-access=free|hdl=2263/20792}}</ref>

Assuming a vaccine is 100% effective, then the equation used for calculating the herd immunity threshold can be used for calculating the vaccination level needed to eliminate a disease, written as ''V''<sub>c</sub>.<ref name=pmid21427399/> Vaccines are usually imperfect however, so the effectiveness, ''E'', of a vaccine must be accounted for:

: <math> V_c=\frac {1 - \frac {1} {R_0}}{E}. </math>

From this equation, it can be observed that if ''E'' is less than (1 − 1/''R''<sub>0</sub>), then it is impossible to eliminate a disease, even if the entire population is vaccinated.<ref name=pmid21427399/> Similarly, waning vaccine-induced immunity, as occurs with [[DPT vaccine#Combination vaccines with acellular pertussis|acellular pertussis vaccines]], requires higher levels of booster vaccination to sustain herd immunity.<ref name=pmid21427399/><ref name=pmid25560446/> If a disease has ceased to be endemic to a population, then natural infections no longer contribute to a reduction in the fraction of the population that is susceptible. Only vaccination contributes to this reduction.<ref name=pmid15627236/> The relation between vaccine coverage and effectiveness and disease incidence can be shown by subtracting the product of the effectiveness of a vaccine and the proportion of the population that is vaccinated, ''p''<sub>v</sub>, from the herd immunity threshold equation as follows:
[[File:Measles cases coverage eastern mediterranean.jpg|right|thumb|Measles vaccine coverage and reported measles cases in [[Eastern Mediterranean]] countries. As coverage increased, the number of cases decreased.]]
: <math> \left(1 - \frac {1} {R_0}\right) - (E \times p_v). </math>

It can be observed from this equation that, all other things being equal ("''[[ceteris paribus]]''"), any increase in either vaccine coverage or vaccine effectiveness, including any increase in excess of a disease's HIT, further reduces the number of cases of a disease.<ref name=pmid15627236/> The rate of decline in cases depends on a disease's ''R''<sub>0</sub>, with diseases with lower ''R''<sub>0</sub> values experiencing sharper declines.<ref name=pmid15627236/>

Vaccines usually have at least one [[contraindication]] for a specific population for medical reasons, but if both effectiveness and coverage are high enough then herd immunity can protect these individuals.<ref name=pmid23910028/><ref name=pmid22773718/><ref name=tulvar/> Vaccine effectiveness is often, but not always, adversely affected by passive immunity,<ref>{{cite journal|vauthors=Leuridan E, Sabbe M, Van Damme P|title=Measles outbreak in Europe: susceptibility of infants too young to be immunized|journal=Vaccine|volume=30|issue=41|pages=5905–13|date=September 2012|pmid=22841972|doi=10.1016/j.vaccine.2012.07.035}}</ref><ref>{{cite journal|vauthors=Hodgins DC, Shewen PE|title=Vaccination of neonates: problem and issues|journal=Vaccine|volume=30|issue=9|pages=1541–59|date=February 2012|pmid=22189699|doi=10.1016/j.vaccine.2011.12.047}}</ref> so additional doses are recommended for some vaccines while others are not administered until after an individual has lost his or her passive immunity.<ref name=pmid21293327/><ref name=tulvar/>

===Passive immunity===
{{Main|Passive immunity}}

Individual immunity can also be gained passively, when [[Antibody|antibodies]] to a pathogen are transferred from one individual to another. This can occur naturally, whereby [[Mother#Biological mother|maternal]] antibodies, primarily [[immunoglobulin G]] antibodies, are transferred across the [[Placenta#Immunity|placenta]] and in [[colostrum]] to fetuses and newborns.<ref name=pmid20956021>{{cite journal|vauthors=Chucri TM, Monteiro JM, Lima AR, Salvadori ML, Kfoury JR, Miglino MA|title=A review of immune transfer by the placenta|journal=Journal of Reproductive Immunology|volume=87|issue=1–2|pages=14–20|date=December 2010|pmid=20956021|doi=10.1016/j.jri.2010.08.062}}</ref><ref>{{cite journal|vauthors=Palmeira P, Quinello C, Silveira-Lessa AL, Zago CA, Carneiro-Sampaio M|title=IgG placental transfer in healthy and pathological pregnancies|journal=Clinical & Developmental Immunology|volume=2012|pages=985646|year=2012|pmid=22235228|pmc=3251916|doi=10.1155/2012/985646|doi-access=free}}</ref> Passive immunity can also be gained artificially, when a susceptible person is injected with antibodies from the [[Serum (blood)|serum]] or [[Blood plasma|plasma]] of an immune person.<ref name=pville/><ref name=parija>{{cite book|vauthors=Parija SC|year=2014|title=Textbook of Microbiology & Immunology|url=https://books.google.com/books?id=bA9tAwAAQBAJ&pg=PA88|publisher=Elsevier Health Sciences|pages=88–89|isbn=978-8131236246|access-date=30 March 2015|archive-date=1 May 2021|archive-url=https://web.archive.org/web/20210501053035/https://books.google.com/books?id=bA9tAwAAQBAJ&pg=PA88|url-status=live}}</ref>

Protection generated from passive immunity is immediate, but wanes over the course of weeks to months, so any contribution to herd immunity is temporary.<ref name=ska/><ref name=pville/><ref name=gph>{{cite book|vauthors=Detels R, Gulliford M, Karim QA, Tan CC|year=2015|title=Oxford Textbook of Global Public Health|url=https://books.google.com/books?id=_ehcBgAAQBAJ&pg=PA1490|publisher=Oxford University Press|page=1490|isbn=978-0199661756|access-date=30 March 2015|archive-date=1 May 2021|archive-url=https://web.archive.org/web/20210501052956/https://books.google.com/books?id=_ehcBgAAQBAJ&pg=PA1490|url-status=live}}</ref> For diseases that are especially severe among fetuses and newborns, such as influenza and tetanus, pregnant women may be immunized in order to transfer antibodies to the child.<ref name=pmid23910028/><ref>{{cite journal|vauthors=Demicheli V, Barale A, Rivetti A|title=Vaccines for women for preventing neonatal tetanus|journal=The Cochrane Database of Systematic Reviews|issue=7|pages=CD002959|date=July 2015|volume=2015 |pmid=26144877|pmc=7138051|doi=10.1002/14651858.CD002959.pub4}}</ref><ref>{{cite journal|vauthors=Swamy GK, Garcia-Putnam R|title=Vaccine-preventable diseases in pregnancy|journal=American Journal of Perinatology|volume=30|issue=2|pages=89–97|date=February 2013|pmid=23271378|doi=10.1055/s-0032-1331032|s2cid=206342684 }}</ref> In the same way, high-risk groups that are either more likely to experience infection, or are more likely to develop complications from infection, may receive antibody preparations to prevent these infections or to reduce the severity of symptoms.<ref name=parija/>