Editing Herd immunity (section)

==Effects==
===Protection of those without immunity===
[[File:WHO EN Herd Immunity Illustration 02 8Dec2020.png|thumb|Herd immunity protects vulnerable communities.]]
Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated.<ref name=pmid23910028>{{cite journal|vauthors=Munoz FM|title=Maternal immunization: an update for pediatricians|journal=Pediatric Annals|volume=42|issue=8|pages=153–8|date=August 2013|pmid=23910028|doi=10.3928/00904481-20130723-09}}</ref><ref name="ofg">{{cite web |title=Herd Immunity |url=http://vk.ovg.ox.ac.uk/herd-immunity |url-status=live |archive-url=https://web.archive.org/web/20190802220355/http://vk.ovg.ox.ac.uk/herd-immunity |archive-date=2 August 2019 |access-date=12 December 2017 |publisher=Oxford Vaccine Group, University of Oxford}}</ref><ref name=pmid24868544>{{cite journal|vauthors=Cesaro S, Giacchino M, Fioredda F, Barone A, Battisti L, Bezzio S, Frenos S, De Santis R, Livadiotti S, Marinello S, Zanazzo AG, Caselli D|display-authors=6|title=Guidelines on vaccinations in paediatric haematology and oncology patients|journal=BioMed Research International|volume=2014|pages=707691|date=2014|pmid=24868544|pmc=4020520|doi=10.1155/2014/707691|doi-access=free}}</ref> Newborn infants are too young to receive many vaccines, either for safety reasons or because [[passive immunity]] renders the vaccine ineffective.<ref name=pmid21293327>{{cite journal|title=General recommendations on immunization --- recommendations of the Advisory Committee on Immunization Practices (ACIP)|journal=MMWR. Recommendations and Reports|volume=60|issue=2|pages=1–64|date=January 2011|pmid=21293327|author1=National Center for Immunization Respiratory Diseases}}</ref> Individuals who are immunodeficient due to [[HIV/AIDS]], [[lymphoma]], [[leukemia]], [[bone marrow]] cancer, an impaired [[spleen]], [[chemotherapy]], or [[Radiation therapy|radiotherapy]] may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.<ref name=ofg/><ref name=pmid24868544/><ref name=pmid21293327/><ref name=pmid22773718>{{cite journal|vauthors=Wolfe RM|title=Update on adult immunizations|journal=Journal of the American Board of Family Medicine|volume=25|issue=4|pages=496–510|year=2012|pmid=22773718|doi=10.3122/jabfm.2012.04.100274|doi-access=free}}</ref>

A portion of those vaccinated may not develop long-term immunity.<ref name=pmid21427399/><ref>{{cite journal|vauthors=Esposito S, Bosis S, Morlacchi L, Baggi E, Sabatini C, Principi N|title=Can infants be protected by means of maternal vaccination?|journal=Clinical Microbiology and Infection|volume=18|issue=Suppl 5|pages=85–92|date=October 2012|pmid=22862749|doi=10.1111/j.1469-0691.2012.03936.x|doi-access=free}}</ref><ref name=rakel2>{{cite book|vauthors=Rakel D, Rakel RE|year=2015|title=Textbook of Family Medicine|url=https://books.google.com/books?id=8huMBgAAQBAJ&pg=PA99|publisher=Elsevier Health Sciences|pages=99, 187|isbn=978-0323313087|access-date=30 March 2015|archive-date=1 May 2021|archive-url=https://web.archive.org/web/20210501052904/https://books.google.com/books?id=8huMBgAAQBAJ&pg=PA99|url-status=live}}</ref> Vaccine [[contraindications]] may prevent certain individuals from being vaccinated.<ref name=pmid24868544/> In addition to not being immune, individuals in one of these groups may be at a greater risk of developing complications from infection because of their medical status, but they may still be protected if a large enough percentage of the population is immune.<ref name=ofg/><ref name=pmid24868544/><ref name=rakel2/><ref name=tulvar>{{cite book|vauthors=Tulchinsky TH, Varavikova EA|year=2014|title=The New Public Health: An Introduction for the 21st Century|url=https://books.google.com/books?id=2hg2IxB9WngC&pg=PA163|publisher=Academic Press|pages=163–82|isbn=978-0124157675|access-date=30 March 2015|archive-date=1 May 2021|archive-url=https://web.archive.org/web/20210501053021/https://books.google.com/books?id=2hg2IxB9WngC&pg=PA163|url-status=live}}</ref>

High levels of immunity in one age group can create herd immunity for other age groups.<ref name=pmid21604922/> Vaccinating adults against [[pertussis]] reduces pertussis incidence in infants too young to be vaccinated, who are at the greatest risk of complications from the disease.<ref name=pmid25560446>{{cite journal|vauthors=McGirr A, Fisman DN|title=Duration of pertussis immunity after DTaP immunization: a meta-analysis|journal=Pediatrics|volume=135|issue=2|pages=331–43|date=February 2015|pmid=25560446|doi=10.1542/peds.2014-1729|url=http://pdfs.semanticscholar.org/b2d8/0ba442e2834148d6f98b49b18bd5808e1348.pdf|archive-url=https://web.archive.org/web/20190303090923/http://pdfs.semanticscholar.org/b2d8/0ba442e2834148d6f98b49b18bd5808e1348.pdf|url-status=dead|archive-date=2019-03-03|s2cid=8273985}}</ref><ref>{{cite journal|vauthors=Zepp F, Heininger U, Mertsola J, Bernatowska E, Guiso N, Roord J, Tozzi AE, Van Damme P|display-authors=6|title=Rationale for pertussis booster vaccination throughout life in Europe|journal=The Lancet. Infectious Diseases|volume=11|issue=7|pages=557–70|date=July 2011|pmid=21600850|doi=10.1016/S1473-3099(11)70007-X }}</ref> This is especially important for close family members, who account for most of the transmissions to young infants.<ref name=pmid21604922/><ref name=rakel2/> In the same manner, children receiving vaccines against pneumococcus reduces pneumococcal disease incidence among younger, unvaccinated siblings.<ref name=pmid22862432>{{cite journal|vauthors=Pittet LF, Posfay-Barbe KM|title=Pneumococcal vaccines for children: a global public health priority|journal=Clinical Microbiology and Infection|volume=18|issue=Suppl 5|pages=25–36|date=October 2012|pmid=22862432|doi=10.1111/j.1469-0691.2012.03938.x|doi-access=free}}</ref> Vaccinating children against pneumococcus and rotavirus has had the effect of reducing [[Streptococcus pneumoniae|pneumococcus]]- and [[rotavirus]]-attributable hospitalizations for older children and adults, who do not normally receive these vaccines.<ref name=pmid22862432/><ref>{{cite journal|vauthors=Nakagomi O, Iturriza-Gomara M, Nakagomi T, Cunliffe NA|title=Incorporation of a rotavirus vaccine into the national immunisation schedule in the United Kingdom: a review|journal=Expert Opinion on Biological Therapy|volume=13|issue=11|pages=1613–21|date=November 2013|pmid=24088009|doi=10.1517/14712598.2013.840285|s2cid=5405583}}</ref><ref>{{cite journal|vauthors=Lopman BA, Payne DC, Tate JE, Patel MM, Cortese MM, Parashar UD|title=Post-licensure experience with rotavirus vaccination in high and middle income countries; 2006 to 2011|journal=Current Opinion in Virology|volume=2|issue=4|pages=434–42|date=August 2012|pmid=22749491|doi=10.1016/j.coviro.2012.05.002|url=https://zenodo.org/record/1258865|access-date=5 July 2019|archive-date=16 October 2019|archive-url=https://web.archive.org/web/20191016205411/https://zenodo.org/record/1258865|url-status=live}}</ref> [[Influenza]] (flu) is more severe in the elderly than in younger age groups, but [[influenza vaccine]]s lack effectiveness in this demographic due to a waning of the immune system with age.<ref name=pmid21604922/><ref name=pmid25003085>{{cite journal|vauthors=Kim TH|title=Seasonal influenza and vaccine herd effect|journal=Clinical and Experimental Vaccine Research|volume=3|issue=2|pages=128–32|date=July 2014|pmid=25003085|pmc=4083064|doi=10.7774/cevr.2014.3.2.128}}</ref> The prioritization of school-age children for seasonal flu immunization, which is more effective than vaccinating the elderly, however, has been shown to create a certain degree of protection for the elderly.<ref name=pmid21604922/><ref name=pmid25003085/>

For [[sexually transmitted infection]]s (STIs), high levels of immunity in heterosexuals of one sex induces herd immunity for heterosexuals of both sexes.<ref name="pmid15627236">{{cite journal |vauthors=Garnett GP |date=February 2005 |title=Role of herd immunity in determining the effect of vaccines against sexually transmitted disease |journal=The Journal of Infectious Diseases |volume=191 |issue=Suppl 1 |pages=S97-106 |doi=10.1086/425271 |pmid=15627236 |doi-access=free}}</ref><ref name=pmid22219162>{{cite journal|vauthors=Lowy DR, Schiller JT|title=Reducing HPV-associated cancer globally|journal=Cancer Prevention Research|volume=5|issue=1|pages=18–23|date=January 2012|pmid=22219162|pmc=3285475|doi=10.1158/1940-6207.CAPR-11-0542}}</ref><ref name=pmid23391351>{{cite journal|vauthors=Lenzi A, Mirone V, Gentile V, Bartoletti R, Ficarra V, Foresta C, Mariani L, Mazzoli S, Parisi SG, Perino A, Picardo M, Zotti CM|display-authors=6|title=Rome Consensus Conference - statement; human papilloma virus diseases in males|journal=BMC Public Health|volume=13|pages=117|date=February 2013|pmid=23391351|pmc=3642007|doi=10.1186/1471-2458-13-117|doi-access=free}}</ref> Vaccines against STIs that are targeted at heterosexuals of one sex result in significant declines in STIs in heterosexuals of both sexes if vaccine uptake in the target sex is high.<ref name=pmid22219162/><ref name=pmid23391351/><ref name=pmid21962468>{{cite journal|vauthors=Garland SM, Skinner SR, Brotherton JM|title=Adolescent and young adult HPV vaccination in Australia: achievements and challenges|journal=Preventive Medicine|volume=53|issue=Suppl 1|pages=S29-35|date=October 2011|pmid=21962468|doi=10.1016/j.ypmed.2011.08.015}}</ref> Herd immunity from female vaccination does not, however, extend to males who have sex with males.<ref name=pmid23391351/> High-risk behaviors make eliminating STIs difficult because, even though most infections occur among individuals with moderate risk, the majority of transmissions occur because of individuals who engage in high-risk behaviors.<ref name=pmid15627236/> For this reason, in certain populations it may be necessary to immunize high-risk individuals regardless of sex.<ref name=pmid15627236/><ref name=pmid23391351/>

===Evolutionary pressure and serotype replacement===
Herd immunity itself acts as an [[evolutionary pressure]] on pathogens, influencing [[viral evolution]] by encouraging the production of novel strains, referred to as escape mutants, that are able to evade herd immunity and infect previously immune individuals.<ref name=pmid24175217>{{Cite journal|pmid=24175217|pmc=3782273|year=2012|last1=Rodpothong|first1=P|title=Viral evolution and transmission effectiveness|journal=World Journal of Virology|volume=1|issue=5|pages=131–34|last2=Auewarakul|first2=P|doi=10.5501/wjv.v1.i5.131 |doi-access=free }}</ref><ref name=pmid23330954>{{Cite journal|pmid=23330954|year=2013|last1=Corti|first1=D|title=Broadly neutralizing antiviral antibodies|journal=Annual Review of Immunology|volume=31|pages=705–42|last2=Lanzavecchia|first2=A|doi=10.1146/annurev-immunol-032712-095916}}</ref> The evolution of new strains is known as [[serotype]] replacement, or serotype shifting, as the [[prevalence]] of a specific serotype declines due to high levels of immunity, allowing other serotypes to replace it.<ref name="pmid21492929">{{Cite journal|last1=Weinberger|first1=D. M.|last2=Malley|first2=R|last3=Lipsitch|first3=M|year=2011|title=Serotype replacement in disease after pneumococcal vaccination|journal=The Lancet|volume=378|issue=9807|pages=1962–73|doi=10.1016/S0140-6736(10)62225-8|pmc=3256741|pmid=21492929}}</ref><ref name="pmid22903767">{{Cite journal|last1=McEllistrem|first1=M. C.|last2=Nahm|first2=M. H.|year=2012|title=Novel pneumococcal serotypes 6C and 6D: Anomaly or harbinger|journal=Clinical Infectious Diseases|volume=55|issue=10|pages=1379–86|doi=10.1093/cid/cis691|pmc=3478140|pmid=22903767}}</ref>

At the molecular level, viruses escape from herd immunity through [[antigenic drift]], which is when [[mutation]]s accumulate in the portion of the [[Virus#Genome|viral genome]] that encodes for the virus's surface [[antigen]], typically a protein of the virus [[capsid]], producing a change in the viral [[epitope]].<ref name=pmid21310617>{{cite journal|vauthors=Bull RA, White PA|title=Mechanisms of GII.4 norovirus evolution|journal=Trends in Microbiology|volume=19|issue=5|pages=233–40|date=May 2011|pmid=21310617|doi=10.1016/j.tim.2011.01.002}}</ref><ref name=pmid24232370>{{cite journal|vauthors=Ramani S, Atmar RL, Estes MK|title=Epidemiology of human noroviruses and updates on vaccine development|journal=Current Opinion in Gastroenterology|volume=30|issue=1|pages=25–33|date=January 2014|pmid=24232370|pmc=3955997|doi=10.1097/MOG.0000000000000022}}</ref> Alternatively, the reassortment of separate viral genome segments, or [[antigenic shift]], which is more common when there are more strains in circulation, can also produce new [[serotype]]s.<ref name=pmid24175217/><ref name=pmid23124938>{{Cite book|pmid=23124938|year=2013|vauthors=Pleschka S|title=Swine Influenza|volume=370|pages=1–20|doi=10.1007/82_2012_272|chapter=Overview of Influenza Viruses|series=Current Topics in Microbiology and Immunology|isbn=978-3642368707}}</ref> When either of these occur, [[memory T cell]]s no longer recognize the virus, so people are not immune to the dominant circulating strain.<ref name=pmid24232370/><ref name=pmid23124938/> For both influenza and [[norovirus]], epidemics temporarily induce herd immunity until a new dominant strain emerges, causing successive waves of epidemics.<ref name=pmid21310617/><ref name=pmid23124938/> As this evolution poses a challenge to herd immunity, [[Neutralizing antibody#Broadly neutralizing antibodies|broadly neutralizing antibodies]] and "universal" vaccines that can provide protection beyond a specific serotype are in development.<ref name=pmid23330954/><ref>{{cite journal|vauthors=Han T, Marasco WA|title=Structural basis of influenza virus neutralization|journal=Annals of the New York Academy of Sciences|volume=1217|issue=1|pages=178–90|date=January 2011|pmid=21251008|pmc=3062959|doi=10.1111/j.1749-6632.2010.05829.x|bibcode=2011NYASA1217..178H}}</ref><ref>{{cite journal|vauthors=Reperant LA, Rimmelzwaan GF, Osterhaus AD|title=Advances in influenza vaccination|journal=F1000Prime Reports|volume=6|pages=47|year=2014|pmid=24991424|pmc=4047948|doi=10.12703/p6-47 |doi-access=free }}</ref>

Initial vaccines against ''[[Streptococcus pneumoniae]]'' significantly reduced nasopharyngeal carriage of vaccine serotypes (VTs), including [[Antimicrobial resistance|antibiotic-resistant]] types,<ref name=pmid22862432/><ref>{{cite journal|vauthors=Dagan R|title=Impact of pneumococcal conjugate vaccine on infections caused by antibiotic-resistant Streptococcus pneumoniae|journal=Clinical Microbiology and Infection|volume=15|issue=Suppl 3|pages=16–20|date=April 2009|pmid=19366365|doi=10.1111/j.1469-0691.2009.02726.x|doi-access=free}}</ref> only to be entirely offset by increased carriage of non-vaccine serotypes (NVTs).<ref name=pmid22862432/><ref name=pmid21492929/><ref name=pmid22903767/> This did not result in a proportionate increase in disease incidence though, since NVTs were less invasive than VTs.<ref name=pmid21492929/> Since then, [[pneumococcal vaccine]]s that provide protection from the emerging serotypes have been introduced and have successfully countered their emergence.<ref name=pmid22862432/> The possibility of future shifting remains, so further strategies to deal with this include expansion of VT coverage and the development of vaccines that use either [[Inactivated vaccine|killed whole-cells]], which have more surface antigens, or proteins present in multiple serotypes.<ref name=pmid22862432/><ref>{{cite journal|vauthors=Lynch JP, Zhanel GG|title=Streptococcus pneumoniae: epidemiology and risk factors, evolution of antimicrobial resistance, and impact of vaccines|journal=Current Opinion in Pulmonary Medicine|volume=16|issue=3|pages=217–25|date=May 2010|pmid=20375783|doi=10.1097/MCP.0b013e3283385653|s2cid=205784538}}</ref>

===Eradication of diseases===
[[File:Rinderpest milk fever.jpg|right|thumb|A cow with [[rinderpest]] in the "[[milk fever]]" position, 1982. The last confirmed case of rinderpest occurred in [[Kenya]] in 2001, and the disease was officially declared eradicated in 2011.]]

If herd immunity has been established and maintained in a population for a sufficient time, the disease is inevitably eliminated{{snd}}no more endemic transmissions occur.<ref name=ska/> If elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared eradicated.<ref name=cliffsr/> [[Eradication of infectious diseases|Eradication]] can thus be considered the final effect or end-result of [[public health]] initiatives to control the spread of contagious disease.<ref name=cliffsr/><ref name=pmid21604922/> In cases in which herd immunity is compromised, on the contrary, disease outbreaks among the unvaccinated population are likely to occur.<ref>{{cite journal |last1=Alhumaid |first1=Saad |last2=Al Mutair |first2=Abbas |last3=Al Alawi |first3=Zainab |last4=Rabaan |first4=Ali A. |last5=Tirupathi |first5=Raghavendra |last6=Alomari |first6=Mohammed A. |last7=Alshakhes |first7=Aqeel S. |last8=Alshawi |first8=Abeer M. |last9=Ahmed |first9=Gasmelseed Y. |last10=Almusabeh |first10=Hassan M. |last11=Alghareeb |first11=Tariq T. |last12=Alghuwainem |first12=Abdulaziz A. |last13=Alsulaiman |first13=Zainab A. |last14=Alabdulmuhsin |first14=Mohammed A. |last15=AlBuwaidi |first15=Emad A. |last16=Dukhi |first16=Amjad K. Bu |last17=Mufti |first17=Hani N. |last18=Al-Qahtani |first18=Manaf |last19=Dhama |first19=Kuldeep |last20=Al-Tawfiq |first20=Jaffar A. |last21=Al-Omari |first21=Awad |title=Anaphylactic and nonanaphylactic reactions to SARS-CoV-2 vaccines: a systematic review and meta-analysis |journal=Allergy, Asthma & Clinical Immunology |date=December 2021 |volume=17 |issue=1 |pages=109 |doi=10.1186/s13223-021-00613-7|pmid=34656181 |pmc=8520206 |doi-access=free }}</ref>

The benefits of eradication include ending all [[Disease#Morbidity|morbidity]] and mortality caused by the disease, financial savings for individuals, health care providers, and governments, and enabling resources used to control the disease to be used elsewhere.<ref name=cliffsr/> To date, two diseases have been eradicated using herd immunity and vaccination: [[rinderpest]] and [[smallpox]].<ref name=pmid21427399/><ref name=pmid21604922/><ref>{{cite journal|vauthors=Njeumi F, Taylor W, Diallo A, Miyagishima K, Pastoret PP, Vallat B, Traore M|title=The long journey: a brief review of the eradication of rinderpest|journal=Revue Scientifique et Technique|volume=31|issue=3|pages=729–46|date=December 2012|pmid=23520729|doi=10.20506/rst.31.3.2157|doi-access=free}}</ref> Eradication efforts that rely on herd immunity are currently underway for [[poliomyelitis]], though civil unrest and distrust of modern medicine have made this difficult.<ref name=pmid21427399/><ref name=pmid23760373>{{cite journal|vauthors=Smith KA|title=Smallpox: can we still learn from the journey to eradication?|journal=The Indian Journal of Medical Research|volume=137|issue=5|pages=895–9|date=May 2013|pmid=23760373|pmc=3734679}}</ref> [[vaccination policy|Mandatory vaccination]] may be beneficial to eradication efforts if not enough people choose to get vaccinated.<ref name=pmid19197342>{{cite journal|vauthors=Perisic A, Bauch CT|title=Social contact networks and disease eradicability under voluntary vaccination|journal=PLOS Computational Biology|volume=5|issue=2|pages=e1000280|date=February 2009|pmid=19197342|pmc=2625434|doi=10.1371/journal.pcbi.1000280|bibcode=2009PLSCB...5E0280P |doi-access=free }}</ref><ref name=pmid20667876>{{cite journal|vauthors=Fu F, Rosenbloom DI, Wang L, Nowak MA|title=Imitation dynamics of vaccination behaviour on social networks|journal=Proceedings. Biological Sciences|volume=278|issue=1702|pages=42–9|date=January 2011|pmid=20667876|pmc=2992723|doi=10.1098/rspb.2010.1107|url=https://dash.harvard.edu/bitstream/handle/1/8298847/Nowak_VaccinationDilemma.pdf?sequence=1|access-date=4 November 2018|archive-date=4 November 2018|archive-url=https://web.archive.org/web/20181104170304/https://dash.harvard.edu/bitstream/handle/1/8298847/Nowak_VaccinationDilemma.pdf?sequence=1|url-status=live}}</ref><ref>{{cite journal|vauthors=Wicker S, Maltezou HC|title=Vaccine-preventable diseases in Europe: where do we stand?|journal=Expert Review of Vaccines|volume=13|issue=8|pages=979–87|date=August 2014|pmid=24958075|doi=10.1586/14760584.2014.933077|s2cid=23471069}}</ref><ref name=stubborn>{{cite book|vauthors=Fukuda E, Tanimoto J|year=2014|title=Impact of Stubborn Individuals on a Spread of Infectious Disease under Voluntary Vaccination Policy|url=https://books.google.com/books?id=kZwvBQAAQBAJ&pg=PA1|publisher=Springer|pages=1–10|isbn=978-3319133591|access-date=30 March 2015|archive-date=1 May 2021|archive-url=https://web.archive.org/web/20210501053012/https://books.google.com/books?id=kZwvBQAAQBAJ&pg=PA1|url-status=live}}</ref>