General information

The information on these pages should be used to research health risks and to inform the pre-travel consultation. For advice regarding safety and security please check the UK Foreign and Commonwealth Office (FCO) website.

Travellers should ideally arrange an appointment with their health professional at least four to six weeks before travel. However, even if time is short, an appointment is still worthwhile. This appointment provides an opportunity to assess health risks taking into account a number of factors including destination, medical history, and planned activities. For those with pre-existing health problems, an earlier appointment is recommended.

While most travellers have a healthy and safe trip, there are some risks that are relevant to travellers regardless of destination. These may for example include road traffic and other accidents, diseases transmitted by insects or ticks, diseases transmitted by contaminated food and water, sexually transmitted infections, or health issues related to the heat or cold.

All travellers should ensure they have adequate travel health insurance.

A list of useful resources including advice on how to reduce the risk of certain health problems is available below.

Resources

Vaccine recommendations

Details of vaccination recommendations and requirements are provided below.

All Travellers

Travellers should be up to date with routine vaccination courses and boosters as recommended in the UK. These vaccinations include for example measles-mumps-rubella (MMR) vaccine and diphtheria-tetanus-polio vaccine. Country specific diphtheria recommendations are not provided here. Diphtheria tetanus and polio are combined in a single vaccine in the UK. Therefore, when a tetanus booster is recommended for travellers, diphtheria vaccine is also given. Should there be an outbreak of diphtheria in a country, diphtheria vaccination guidance will be provided.

Those who may be at increased risk of an infectious disease due to their work, lifestyle choice, or certain underlying health problems should be up to date with additional recommended vaccines. See the individual chapters of the ‘Green Book’ Immunisation against infectious disease for further details.

Certificate Requirements

Please read the information below carefully, as certificate requirements may be relevant to certain travellers only. For travellers further details, if required, should be sought from their healthcare professional.

Polio

Travellers who intend to visit Nigeria for four weeks or more should be aware that proof of vaccination, given four weeks to 12 months before departure from the country an International Certificate of Vaccination or Prophylaxis (ICVP), may be required on exit. Failure to produce this documentation may result in vaccination at the point of departure, most likely with oral polio vaccine.  See ‘Most Travellers‘ section below for further details.

Yellow fever

  • There is a risk of yellow fever transmission throughout Nigeria  (see ‘Most Travellers’ section below).
  • Under International Health Regulations (2005), a yellow fever vaccination certificate is required from travellers over 1 year of age arriving from countries with risk of yellow fever transmission.
  • According to World Health Organization (WHO), from 11 July 2016 (for all countries), the yellow fever certificate will be valid for the duration of the life of the person vaccinated. As a consequence, a valid certificate, presented by arriving travellers, cannot be rejected on the grounds that more than ten years have passed since the date vaccination became effective as stated on the certificate; and that boosters or revaccination cannot be required.
    See WHO Q&A
  • View the WHO list of countries with risk of yellow fever transmission.

Most Travellers

The vaccines in this section are recommended for most travellers visiting this country. Information on these vaccines can be found by clicking on the blue arrow. Vaccines are listed alphabetically.

Hepatitis A

Hepatitis A is a viral infection transmitted through contaminated food and water or by direct contact with an infectious person. Symptoms are often mild or absent in young children, but the disease becomes more serious with advancing age. Recovery can vary from weeks to months. Following hepatitis A illness immunity is lifelong.

Those at increased risk include travellers visiting friends and relatives, long-stay travellers, and those visiting areas of poor sanitation.

Prevention

All travellers should take care with personal, food and water hygiene.

Hepatitis A vaccination

As hepatitis A vaccine is well tolerated and affords long-lasting protection, it is recommended for all previously unvaccinated travellers.

Hepatitis A in brief

Polio

Polio is caused by one of three types of polio virus and is transmitted by contaminated food and water. Previous infection with one type of polio virus does not protect against other types of the virus.

Those at increased risk include travellers visiting friends and relatives, those in direct contact with an infected person, long-stay travellers, and those visiting areas of poor sanitation.

Polio in Nigeria

This country is infected with wild type polio virus with the potential risk of international spread.

Prevention

All travellers should take care with personal and food and water hygiene.

Polio vaccination
  • All travellers should have completed a polio vaccination course according to the UK schedule.
  • A booster dose of a polio-containing vaccine is recommended for those who have not received a dose within the previous 10 years.

The following additional advice should also be followed until further notice:
  • Travellers to settings with extremely poor hygiene (e.g. refugee camps), or likely to be in close proximity with cases (e.g. healthcare workers), and/or visiting for 6 months or more, are advised to have a booster dose of polio-containing vaccine if they had not received vaccination in the past 12 months.
  • Travellers who intend to visit Nigeria for 4 weeks or more should be aware that proof of vaccination, given 4 weeks to 12 months before departure from the country [an International Certificate of Vaccination or Prophylaxis (ICVP)], may be required on exit. Failure to produce this documentation may result in vaccination at the point of departure, most likely with oral polio vaccine.
  • Immunosuppressed and their household contacts or pregnant individuals who plan to travel to Nigeria for 4 weeks or more should not given oral polio vaccine; such individuals are therefore advised to receive inactivated polio vaccine (IPV) within 1 year before planned departure from Nigeria and to ensure this is recorded on an ICVP.
  • Further information from WHO on the Public Health Emergency of International Concern.
  • Further information from WHO about the cessation of OPV and replacement with IPV 2016.

Polio in brief

Tetanus

Tetanus is caused by a toxin released from Clostridium tetani and occurs worldwide. Tetanus bacteria are present in soil and manure and may be introduced through open wounds such as a puncture wound, burn or scratch.

Prevention

Travellers should thoroughly clean all wounds and seek appropriate medical attention.

Tetanus vaccination
  • Travellers should have completed a primary vaccination course according to the UK schedule.
  • If travelling to a country where medical facilities may be limited, a booster dose of a tetanus-containing vaccine is recommended if the last dose was more than ten years ago even if five doses of vaccine have been given previously.

Country specific information on medical facilities may be found in the ‘health’ section of the FCO foreign travel advice website.

Tetanus in brief

Typhoid

Typhoid is a bacterial infection transmitted through contaminated food and water. Previous typhoid illness may only partially protect against re-infection.

Those at increased risk include travellers visiting friends and relatives, those in contact with an infected person, young children, long-stay travellers, and those visiting areas of poor sanitation.

Prevention

All travellers should take care with personal, food and water hygiene.

Typhoid vaccination
  • Both oral and injectable typhoid vaccinations are available and are recommended for those at increased risk (see above).
  • Vaccination could be considered for other travellers.

Typhoid in brief

Yellow Fever

Yellow fever is a viral infection transmitted by  mosquitoes which  predominantly feed between dawn and dusk, but may also bite at night, especially in the jungle environment. Symptoms may be absent or mild, but in severe cases, it can cause internal bleeding, organ failure and death

Yellow Fever in Nigeria

There is a risk of yellow fever transmission throughout this country (see map below).

Prevention

All travellers should avoid mosquito bites particularly between dawn and dusk.

 Yellow fever vaccination
  • Vaccination is recommended for travellers aged 9 months and older
  • See vaccine recommendation map below

The yellow fever vaccine is not suitable for all travellers, there are specific undesirable effects associated with it. This vaccine is only available at registered yellow fever vaccination centres. Health professionals should carefully assess the risks and benefits of the vaccine, and seek specialist advice if necessary.

Yellow fever in brief

Yellow fever vaccine recommendations in Africa

Map provided by the Travelers’ Health Branch, Centers for Disease Control and Prevention

YF-map-Africa

Current as of July 2017. This map, which aligns with recommendations also published by the World Health Organization (WHO), is an updated version of the 2010 map created by the Informal WHO Working Group on the Geographic Risk of Yellow Fever.

1. Yellow fever (YF) vaccination is generally not recommended in areas where there is low potential for YF virus exposure. However, vaccination might be considered for a small subset of travelers to these areas who are at increased risk for exposure to YF virus because of prolonged travel, heavy exposure to mosquitoes, or inability to avoid mosquito bites. Consideration for vaccination of any traveler must take into account the traveler’s risk of being infected with YF virus, country entry requirements, and individual risk factors for serious vaccine-associated adverse events (e.g. age, immune status).

Some Travellers

The vaccines in this section are recommended for some travellers visiting this country. Information on when these vaccines should be considered can be found by clicking on the arrow. Vaccines are listed alphabetically.

Cholera

Cholera is a bacterial infection transmitted by contaminated food and water. Cholera can cause severe watery diarrhoea although mild infections are common. Most travellers are at low risk.

Prevention

All travellers should take care with personal, food and water hygiene.

Cholera vaccination

This oral vaccine is recommended for those whose activities or medical history put them at increased risk.  This includes:

  • aid workers
  • those going to areas of cholera outbreaks who have limited access to safe water and medical care.
  • those for whom vaccination is considered potentially beneficial.

 Cholera in brief

Hepatitis B

Hepatitis B is a viral infection; it is transmitted by exposure to infected blood or body fluids. This mostly occurs during sexual contact or as a result of blood-to-blood contact (for example from contaminated equipment during medical and dental procedures, tattooing or body piercing procedures, and sharing of intravenous needles). Mothers with the virus can also transmit the infection to their baby during childbirth.

Hepatitis B in Nigeria

2% or more of the population are known or thought to be persistently infected with the hepatitis B virus (intermediate/high prevalence).

Prevention

Travellers should avoid contact with blood or body fluids. This includes:

  • avoiding unprotected sexual intercourse.
  • avoiding tattooing, piercing, public shaving,  and acupuncture (unless sterile equipment is used)
  • not sharing needles or other injection equipment.
  • following universal precautions if working in a medical/dental/high risk setting.

A sterile medical equipment kit may be helpful when travelling to resource poor areas.

Hepatitis B vaccination

Vaccination could be considered for all travellers, and is recommended for those whose activities or medical history put them at increased risk including:

  • those who may have unprotected sex.
  • those who may be exposed to contaminated needles through injecting drug use.
  • those who may be exposed to blood or body fluids through their work (e.g. health workers).
  • those who may be exposed to contaminated needles as a result of having medical or dental care e.g. those with pre-existing medical conditions and those travelling for medical care abroad including those intending to receive renal dialysis overseas.
  • long-stay travellers
  • those who are participating in contact sports.
  • families adopting children from this country.

Hepatitis B in brief

Meningococcal Disease

Meningococcal disease is a bacterial infection transmitted by inhaling respiratory droplets or direct contact with respiratory secretions from an infected person. This is usually following prolonged or frequent close contact. The most common forms of meningococcal disease are meningococcal meningitis (infection of the protective lining around the brain) and septicaemia (blood poisoning).

Those at increased risk include healthcare workers, those visiting friends and relatives and long-stay travellers who have close contact with the local population.

Meningococcal disease in Nigeria

This country lies within the meningitis belt of sub-Saharan Africa.

Prevention

Travellers should avoid, if possible, overcrowded conditions.

Meningococcal disease vaccination

Vaccination is recommended for those whose activities or medical condition put them at increased risk including:

  • healthcare workers
  • those visiting friends and relatives
  • those who live or travel ‘rough’ such as backpackers
  • long-stay travelers who have close contact with the local population
  • those with certain rare immune system problems (complement disorders) and those who do not have a functioning spleen

For travellers at risk, the ACWY conjugate vaccines are recommended.

Meningococcal disease in brief

Rabies

Rabies is a viral infection which is usually transmitted following contact with the saliva of an infected animal most often via a bite, scratch or lick to an open wound or mucous membrane (such as on the eye, nose or mouth). Although many different animals can transmit the virus, most cases follow a bite or scratch from an infected dog. In some parts of the world, bats are an important source of infection.

Rabies symptoms can take some time to develop, but when they do, the condition is almost always fatal.

The risk of exposure is increased by certain activities and length of stay (see below). Children are at increased risk as they are less likely to avoid contact with animals and to report a bite, scratch or lick.

Rabies in Nigeria

Rabies has been reported in domestic and wild animals in this country. Bats may also carry rabies-like viruses.

Prevention
  • Travellers should avoid contact with all animals. Rabies is preventable with prompt post-exposure treatment.
  • Following a possible exposure, wounds should be thoroughly cleansed and an urgent local medical assessment sought, even if the wound appears trivial.
  • Post-exposure treatment and advice should be in accordance with national guidelines.
Rabies vaccination

Pre-exposure vaccinations are recommended for travellers whose activities put them at increased risk including:

  • those at risk due to their work (e.g. laboratory staff working with the virus, those working with animals or health workers who may be caring for infected patients).
  • those travelling to areas where access to post-exposure treatment and medical care is limited.
  • those planning higher risk activities such as running or cycling.
  • long-stay travellers (more than one month).


A full course of pre-exposure vaccines simplifies and shortens the course of post-exposure treatment and removes the need for rabies immunoglobulin which is in short supply world-wide.

Rabies in brief

Tuberculosis (TB)

TB is a bacterial infection transmitted most commonly by inhaling respiratory droplets from an infectious person. This is usually following prolonged or frequent close contact.

Tuberculosis in Nigeria

The average annual incidence of TB is greater than or equal to 40 cases per 100,000 population (further details).

Prevention

Travellers should avoid close contact with individuals known to have infectious pulmonary (lung) TB.

Those at risk during their work (such as healthcare workers) should take appropriate infection control precautions.

Tuberculosis (BCG) vaccination

According to current national guidance, BCG vaccine should be recommended for those at increased risk of developing severe disease and/or of exposure to TB infection e.g. when the average annual incidence of TB is greater than or equal to 40 cases per 100,000 population. See Public Health England’s Immunisation against infectious disease, the ‘Green Book’.

For travellers, BCG vaccine is also recommended for:

  • unvaccinated, children under 16 years of age, who are going to live for more than 3 months in this country. A tuberculin skin test is required prior to vaccination for all children from 6 years of age and may be recommended for some younger children.

  • unvaccinated, tuberculin skin test negative individuals under 35 years of age at risk due to their work such as healthcare workers, prison staff and vets. Healthcare workers may be vaccinated over the age of 35 years following a careful risk assessment.

There are specific contraindications associated with the BCG vaccine and health professionals must be trained to administer this vaccine intradermally (just under the top layer of skin).

Following administration, no further vaccines should be administered in the same limb for 3 months.

The BCG vaccine is given once only, booster doses are not recommended.

Tuberculosis in brief

Malaria

Malaria is a serious illness caused by infection of red blood cells with a parasite called Plasmodium. The disease is transmitted by mosquitoes which predominantly feed between dusk and dawn.

Symptoms usually begin with a fever (high temperature) of 38°C (100°F) or more. Other symptoms may include feeling cold and shivery, headache, nausea, vomiting and aching muscles. Symptoms may appear between eight days and one year after the infected mosquito bite.

Prompt diagnosis and treatment is required as people with malaria can deteriorate quickly. Those at higher risk of malaria, or of severe complications from malaria, include pregnant women, infants and young children, the elderly, travellers who do not have a functioning spleen and those visiting friends and relatives.

Prevention

Travellers should follow an ABCD guide to preventing malaria:

Awareness of the risk – Risk depends on the specific location, season of travel, length of stay, activities and type of accommodation.
Bite prevention – Travellers should take mosquito bite avoidance measures.
Chemoprophylaxis – Travellers should take antimalarials (malaria prevention tablets) if appropriate for the area (see below). No antimalarials are 100% effective but taking them in combination with mosquito bite avoidance measures will give substantial protection against malaria.
Diagnosis – Travellers who develop a fever of 38°C [100°F] or higher more than one week after being in a malaria risk area, or who develop any symptoms suggestive of malaria within a year of return should seek immediate medical care. Emergency standby treatment may be considered for those going to remote areas with limited access to medical attention.

Risk Areas

  • There is a high risk of malaria in Nigeria: atovaquone/proguanil OR doxycycline OR mefloquine recommended

Recommended Antimalarials

The recommended antimalarials are listed below. If these are not suitable please seek further specialist advice.

Please note, the advice for children is different, the dose is based on body weight and some antimalarials are not suitable.

Atovoquone/Proguanil

Atovaquone 250mg/Proguanil 100mg combination preparation:

  • start one to two days before arrival in the malaria risk area
  • for adults, one tablet is taken every day, ideally at the same time of day for the duration of the time in a malaria risk area and daily for seven days after leaving the malaria risk area
  • take with a fatty meal if possible
  • for children paediatric tablets are available and the dose is based on body weight (see table below)

Doxycycline

Doxycycline 100mg:

  • start one to two days before arrival in the malaria risk area
  • adults and children over 12 years of age take 100mg daily, ideally at the same time of day for the duration of the time in a malaria risk area and daily for four weeks after leaving the malaria risk area
  • take with food if possible; avoid taking this drug just before lying down
  • not suitable for children under 12 years of age

Mefloquine

Mefloquine 250mg:

  • this drug is taken weekly, adults take one 250mg tablet each week
  • start two to three weeks before arrival in the malaria risk area and continue weekly until four weeks after leaving the malaria risk area
  • for children the dose is based on the body weight (see table below)

Resources

Other risks

The risks below may be present in all or part of the country and are listed alphabetically.

Dengue

Dengue is a viral infection transmitted by mosquitoes which predominantly feed between dawn and dusk.  It causes a flu-like illness, which can occasionally develop into a more serious life-threatening form of the disease. Severe dengue is rare in travellers.

The mosquitoes that transmit dengue are most abundant in towns, cities and surrounding areas. All travellers to dengue areas are at risk.

Dengue in Nigeria

Dengue is known or has the potential to occur in this country.

Prevention

  • All travellers should avoid mosquito bites particularly between dawn and dusk.
  • There is no vaccination or medication to prevent dengue.

Dengue in brief

Schistosomiasis

Schistosomiasis is a parasitic infection. Schistosoma larvae are released from infected freshwater snails and can penetrate intact human skin following contact with contaminated freshwater. Travellers may be exposed during activities such as wading, swimming, bathing or washing clothes in freshwater streams, rivers or lakes.

Schistosomiasis infection may cause no symptoms, but early symptoms can include a rash and itchy skin (‘swimmer’s itch’), fever, chills, cough, or muscle aches. If not treated, it can cause serious long term health problems such as intestinal or bladder disease.

Schistosomiasis in Nigeria

According to World Health Organization (WHO), cases of schistosomiasis were reported in this country in 2012.

Prevention

  • There is no vaccine or tablets to prevent schistosomiasis.
  • All travellers should avoid wading, swimming, or bathing in freshwater where possible. Swimming in chlorinated water or sea water is not a risk for schistosomiasis.
  • Topical application of insect repellent before exposure to water, or towel drying after accidental exposure to schistosomiasis are not reliable in preventing infection.
  • All travellers who may have been exposed to schistosomiasis should have a medical assessment.

Schistosomiasis in brief

Zika Virus

Zika virus (ZIKV) is a viral infection transmitted by mosquitoes which predominantly feed between dawn and dusk. A small number of cases of sexual transmission of ZIKV have also been reported. Most people infected with ZIKV have no symptoms. When symptoms do occur they are usually mild and short-lived. Serious complications and deaths are not common. However, there is now scientific consensus that Zika virus is a cause of congenital Zika syndrome (microcephaly and other congenital anomalies) and Guillain-Barré syndrome.

Zika virus in Nigeria

This country is considered to have a moderate risk of Zika virus transmission. Pregnant women should consider postponing non-essential travel until after the pregnancy. Details of specific affected areas within this country are not available.

Prevention

  • All travellers should avoid mosquito bites particularly between dawn and dusk.
  • There is no vaccination or medication to prevent ZIKV infection.
  • It is recommended that pregnant women planning to travel to areas a moderate risk of ZIKV transmission should consider postponing non-essential travel until after pregnancy.
  • Women should avoid becoming pregnant while travelling in, and for 8 weeks after leaving an area with active ZIKV transmission or 8 weeks after last possible ZIKV exposure.
  • Couples should follow guidance on prevention of sexual transmission of Zika and avoid conception while travelling and for up to 6 months on return.
  • If a woman develops symptoms compatible with ZIKV infection, it is recommended she avoids becoming pregnant for a further 8 weeks following recovery.
  • Pregnant women who visited this country while pregnant, or who become pregnant within 8 weeks of leaving this country or within 8 weeks after last possible ZIKV exposure, should contact their GP, obstetrician or midwife for further advice, even if they have not been unwell. Further information about when to perform fetal ultrasound scanning, and, if necessary, referral to the local fetal medicine service is available.

Preventing sexual transmission

See detailed guidance on factors to consider when assessing the risk of ZIKV.

Zika virus in brief

Important News

05 Sep 2017

Polio: Public Health Emergency of International Concern- update

An update on the polio Public Health Emergency of International Concern (PHEIC) Read more

03 Aug 2017

Zika virus – update and advice for travellers

Following an expert review, three risk ratings for areas of Zika virus transmission have been defined - Advice for travellers visiting these areas has Read more

09 May 2017

Polio: Public Health Emergency of International Concern- update

An update on the polio Public Health Emergency of International Concern (PHEIC) Read more

25 Nov 2016

Polio: Public Health Emergency of International Concern update

An update on the polio Public Health Emergency of International Concern (PHEIC) Read more

15 Aug 2016

Nigeria: first cases of wild polio reported since 2014

After more than two years without wild poliovirus in Nigeria, the Government reported that two children have been paralysed by the disease in the nort Read more

07 Jul 2016

Changes to Country Information pages: polio vaccination

NaTHNaC changes polio vaccine recommendations for South Sudan following statement from WHO 9th Emergency Committee statement to reduce international s Read more

01 Dec 2015

Circulating vaccine derived polio virus (cVDPV): changes to Country Information pages - polio vaccination

Latest on WHO temporary recommendations to prevent the international spread of cVDPV and update to country status Read more

Outbreaks

08 Nov 2017 Nigeria

As of 27 October 2017, suspected cases of yellow fever continue to be investigated from six states (Abia, Borno, Kwara, Kogi, Plateau, and Zamfara). Of seventeen samples sent to the Pasteur Institute, Senegal, three cases (two from Kogi and one from Kwara) have been confirmed; five specimens have tested negative and nine results are pending.

Human

Vector-Borne

Updates 2

Verified

WHO - Read more

06 Nov 2017 Nigeria

As of 2 November 2017, a total of 116 cases have been reported across 21 states. Confirmed cases (38) are reported from the states of Akwa Ibom, Bayel sa, Delta, Edo, Ekiti, Enugu, Lagos, Rivers and FCT.

Human

Miscellaneous

Updates 2

Verified

State - Read more

31 Oct 2017 Nigeria

Between 19 February and 29 September 2017, a total of 869 cases (264 confirmed) and 119 deaths have been reported. As of 29 September 2017, the outbreak is currently active in nine states: Ondo, Edo, Plateau, Bauchi, Lagos, Ogun, Kaduna, Kwara, and Kogi.

Human

Haemorrhagic fever

Updates 12

Verified

WHO - Read more

19 Oct 2017 Borno. Nigeria

As of 18 October 2017, a total of 5,020 cases (206 confirmed) with 61 deaths have been reported in Borno State since August 2017. The number of cases peaked in September.

Human

Food and water-borne

Updates 2

Verified

State - Read more

28 Sep 2017 Borno. Nigeria

As of 22 September 2017, a total of 3,498 with 53 deaths have been reported in Borno State since 20 August 2017.

Human

Food and water-borne

Updates 3

Verified

WHO - Read more

24 Aug 2017 Nigeria

As of 16 August 2017, a total of 759 suspected cases of hepatitis E (42 confirmed) with four deaths have been reported in the Borno State May-June 2017. The number of cases in the last week of reporting was 26. 

Human

Food and water-borne

Updates 1

Verified

WHO - Read more

21 Jul 2017 Nigeria

As of 7 July 2017, a total of 13,893 suspected cases (92 confirmed) with 79 deaths have been reported during the first six months this year.

Human

Close association

New Post

Verified

State - Read more

13 Jun 2017 Nigeria

From 13 December, 2016 to 2 June, 2017, a total of 14,473 suspected cases of meningiitis and 1,155 deaths have been reported from 25 States. Of the 460 laboratory confirmed cases, 80.6% were Neisseria meningitidis serogroup C. In the last four weeks 24 local government areas have been in alert / epidemic status in three States- Zamfara, Sokoto and Katsina.

Human

Close association

Updates 7

Verified

State - Read more

06 Mar 2017 Borno. Nigeria

As of 3 March 2017, the first case in Borno State since 1969 has been reported.

Human

Haemorrhagic fever

New Post

Verified

WHO - Read more

23 Jan 2017 Sokoto. Nigeria

As of 17 January 2017, a case of circulating vaccine-derived poliovirus type 2 (cVDPV2) with onset of paralysis 26 October 2016 has been reported in Bodinga Local Government Area, Sokoto state.

Human

Food and water-borne

New Post

Verified

WHO - Read more

21 Dec 2016 Nigeria

As of 17 December 2016, the first reported cases have been confirmed in guinea fowls, turkeys and pigeons.