The aim of cancer vaccines is to stimulate the immune system to be able to recognise cancer cells as abnormal and destroy them.
Each type of cancer vaccine works on the same basic idea that the vaccine, which contains tumor cells or antigens, stimulates the patient’s immune system, which produces special cells that kill cancer cells and prevent relapses of cancer, in short they work by stimulating or restoring the immune system’s ability to fight infections and diseases.
Some cancers are caused by viruses, and vaccines that help to protect against such viral infections may actually help in preventing certain cancers. Viruses known to cause human cancers are like HPV infections causing cervical, vaginal, vulval, anal and penile cancers. HBV and HCV causing liver cancer, HTLV1 causing T cell leukemia/lymphoma
Cancer vaccines can be broadly classified as cancer prevention vaccines and cancer treatment vaccines.
Cancer prevention vaccines
Preventive or prophylactic vaccines prevent cancer from developing in healthy people.
The Food and Drug Administration (FDA) has approved two types of vaccines for cancer prevention: These are
HPV stands for human papillomavirus which is a group of 200 related viruses that can cause cancer and some diseases in men and women, and more than 100 types of HPV types exist. HPV is usually spread through oral, anal or vaginal sex. Certain HPV types cause genital warts and others cause common warts on the fingers, near the fingernails, or on the hands. Human papilloma viruses usually are spread by direct skin contact. Other types of HPV are present in the mucous membranes which are moist layers that line the openings in the body and organs such as mouth, throat and vagina.
Different types of HPV are assigned specific numbers. HPV 6 and 11 are the types that are low-risk and account for 90 percent of genital warts. HPV 11 can cause changes in the cells of the cervix.
HPV 16 and HPV 18 are two types of high risk viruses. HPV 16 is the most common type that does not result in noticeable symptoms but accounts for about half of cervical cancer cases worldwide. Both HPV 16 and 18 together account for more than 70% cervical cancer cases. Getting the HPV vaccine may help prevent HPV 6. The vaccine also offers some protection from HPV 11.
Three different vaccines, which vary in the number of HPV types they contain and target are available.
Quadrivalent HPV vaccine (Gardasil) targets HPV types 6, 11, 16, and 18.
9-valent vaccine (Gardasil 9) targets the same HPV types as the quadrivalent vaccine (6, 11, 16 and 18) as well as types 31, 33, 45, 52, and 58. However, this vaccine is not available anywhere except in the US.
Bivalent vaccine (Cervarix) targets HPV types 16 and 18.
The recommended age group is between 9 and 26 years.
Schedule for HPV vaccine
Routine HPV vaccination is recommended at 11 to 12 years. It can be administered starting at 9 years of age. For adolescents and adults aged 13 to 26 years who have not been previously vaccinated or who have not completed the vaccine series. This vaccine is given in 3 doses with each dose containing 0.5-mL of the vaccine at 0, 1, and 6 months intervals.
The vaccine schedule as recommended by the Centers for Disease Control and Prevention (CDC) is as below:
For children in the age group of 9 to 14
- A 2-dose schedule is recommended for people who get the first dose before the age of 15.
- The minimum age for the first dose of HPV vaccine is 9 years.
- Two doses of the vaccine are recommended by CDC for 11 or 12 year old adolescents.
- The second dose is to be given six to twelve months after giving the first dose (0, 6–12 schedule).
- The minimum gap between first and second dose should be five months.
- If the vaccine’s second dose is given within a gap of five months, a third dose needs to be given after five months from the first dose and twelve weeks after giving the second dose.
For the age group between 15 and 26 years
- CDC recommends a 3-dose schedule for those who got their first dose of the vaccine on or after 15 years of age and for those who have immunocompromising conditions.
- The second dose in this series has to be given one to two months after giving the first dose, while the third one is given six months post the first dose (0, 1–2, 6 schedule).
- Between first and second dosage, there should be a 4 weeks interval while there should be a gap of 12 weeks at a minimum between second and third doses. The gap between first and the third dose should be five months.
Limitations of HPV vaccine
Not effective against previously contracted HPV
Studies show Gardasil and Cervarix are not beneficial in treating previously contracted HPV. These vaccines do not prevent other sexually transmitted diseases either.
In India, two HPV vaccines are available currently including Bivalent (Cervarix) and Quadrivalent (Gardasil) vaccines.The relatively higher cost of these may be prohibitive which can reduce access to the vaccine across all regions.
Does not protect against all HPV related cancers
The vaccines are only effective against some HPV linked cervical cancers. Getting regular screening tests is still important even if you are vaccinated.
Hepatitis B vaccine
Hepatitis B is a liver disease that is caused by a virus called hepatitis B virus. Acute hepatitis B is a term used to refer to a period of the first six months of infection with hepatitis B virus. In some people the hepatitis B infection becomes long term or “chronic,” where the virus remains active after 6 months. Over time, the active virus can lead to serious health problems including liver cancer.
Who should take the hepatitis B vaccine?
Hepatitis B vaccine is recommended universally for all newborn children, and unvaccinated adolescents below the age of 19 years, and for sexually active adults who have multiple sexual partners, use needles or have a partner with known hepatitis B infection.
Schedule for hepatitis B vaccine
Three shots are given to prevent hepatitis B infection. For infants, the schedule is 6, 10, 14 weeks or alternatively, 0, 6, 14 weeks. For children and adults, the schedule is 0, 1, 6 months.
Cancer treatment vaccines
Also known as therapeutic vaccines, treat an existing cancer by strengthening the body’s natural defenses against the cancer.
These vaccines help in preventing the cancer relapse, help in destroying the cancer cells, and prevent the growth or spread of the tumor.
Some cancer treatment vaccines are made up of cancer cells, parts of cancer cells, or specific proteins in the cancer cells. While in some cases, the patient’s immune cells are taken and exposed to the cancer cells in the lab to create the vaccine. The vaccine is then injected into the patient’s body to increase the immune response against the cancer cells.
There are many cancer vaccines still under trials. FDA approved cancer treatment vaccines include sipuleucel-T for metastatic hormone-refractory prostate cancer. Bacillus Calmette-Guérin (BCG) vaccine for early-stage bladder cancer. Talimogene laherparepvec (T-VEC) approved to treat melanoma.
This vaccine is approved to treat prostate cancer that is not responsive to hormone therapy.
This vaccine uses the patient’s own immune system to recognize and combat the cancer.
To prepare this vaccine, immune system cells from the blood of the patient are taken and sent to a lab where they are pulsed with a peptide (PAP-GM-CSF fusion protein) derived from prostatic acid phosphatase and turned into modified dendritic cells which are specialized immune cells. These cells are infused back to the patient and repeated two times with a gap of 2 weeks. The immune system cells help attack cancer cells in the prostate.
The limitations of this vaccine is that it does not cure prostate cancer. However, it can help extend lifespan by several months.
BCG vaccine is a live attenuated form of Mycobacterium bovis, most commonly used agent for intravesical therapy for early stage urinary bladder cancer. It usually follows a procedure called transurethral resection of bladder tumour (TURBT). It’s intended to help prevent recurrence.
Limitations of cancer treatment vaccines
- Larger tumors are difficult to destroy using only vaccines. This is the reason why cancer vaccines are given in some instances along with other treatments.
- People with a weak immune system may not respond to vaccines. People who are chronically ill or older may not have a strong immune response after receiving vaccination. This can reduce the efficacy of the vaccine. Besides,certain treatments for cancer may damage the immune system and hamper its ability to respond.
- Cancer cells grow by suppressing the immune system. This makes cancer treatment vaccines hard to work as the main goal of these vaccines is to boost the immune responses. Researchers are working on using adjuvants in vaccines to fix this problem.
Cancer treatment vaccines work best in prevention of certain cancers like cervical cancers. hepatitis B linked liver cancer and in treating early-stage urinary bladder and prostate cancers . Consult a leading oncologist in your area to know more about cancer vaccines.