How Do Vaccines Work, and Do They Help?

How do vaccines work, and do they help?

Why in News?

  • India recently began vaccination against the novel coronavirus disease.

How do vaccines work; and do they help?

  • A vaccine is a substance that resembles the disease-causing agent (called pathogen, the coronavirus in this case) that trains the body’s immune system and creates a ‘memory’.
  • When the pathogen infects in future, this memory is rapidly deployed to destroy it and prevent disease.
  • Evidence shows that incidence of an infectious disease goes down rapidly following the deployment of vaccines against it; several human diseases are now vaccine preventable.
  • Besides the eradication of smallpox and near eradication of polio, vaccines have resulted in the prevention of over 20 other life-threatening diseases, avoiding an estimated 2-3 million deaths annually.
  • India’s Universal Immunization Programme, among the world’s largest, immunizes about 26 million children every year.
  • It is estimated that every dollar spent on childhood vaccines adds $44 to the economy by ensuring that children grow up to be healthy adults.

Vaccines take long to develop. How could the Covid-19 vaccines be ready so soon?

  • It can indeed take several years to develop a vaccine.
  • After a proof of concept has been established in research laboratories, controlled manufacturing processes are developed to make stable and highly pure products that are tested on animals and then on humans for safety and effectiveness.
  • Clinical trials in human beings are carried out in three phases to seek specific answers.
  • Phase 1: is done in typically 20-100 healthy volunteers to see if the vaccine is safe, if it appears to work, whether there are any serious side effects, and if these are related to the size of the dose.
  • Phase 2: uses several hundred volunteers to determine the most common short-term side effects, and how well the immune system responds to the vaccine — what is known as ‘immunogenicity’.
  • Phase 3: involves thousands of volunteers in a blinded manner to compare those who get the vaccine to those who don’t (they get a placebo or dummy) to re-confirm safety, serious side-effects if any, and most importantly, whether the vaccine is efficacious in preventing infection and/or disease.

There are several reasons why Covid-19 vaccines were developed so quickly.

  • Scientific information has been shared openly, and it helped that the virus was similar to the SARS-CoV-1 and MERS viruses on which considerable work had already been done.
  • Regulators have also allowed parallel phases of clinical testing and reviewing of data to expedite the process.
  • Large investments by governments and innovative financial models allowed pharmaceutical companies to work on developing the vaccine without having to absorb all the financial risk.
  • Another crucial reason was the use of every available vaccine platform to produce a Covid-19 vaccine, including those that had so far not produced a vaccine for humans.
  • The vaccines made by Pfizer/BioNTech and Moderna both directly deliver an mRNA fragment into human cells to produce the viral Spike protein which raises anti-viral immunity.
  • This technology had been in development for about a decade for anti-cancer vaccines.
  • Similarly, non-replicating viral vectors had been in development for years.
  • An experimental adenovirus-based Ebola vaccine was used to vaccinate about 60,000 people in West Africa during the 2014-16 Ebola outbreak.
  • Researchers at Oxford University in the United Kingdom had been using the chimpanzee adenovirus platform for several experimental vaccines, which was quickly repurposed to develop a Covid-19 vaccine.
  • Finally, vaccines based on inactivated viruses is a time-tested method, which has been used in the ICMR/Bharat Biotech vaccine as also in at least three vaccines from China.

Limitations of each platform

  • The mRNA is a fragile molecule that requires protection, including frozen storage, which complicates its rollout logistics.
  • Viral vector vaccines are more stable (2 to 8 deg C storage), but the same vector cannot be used for another disease in the same person because anti-vector immunity will make it ineffective.
  • Though inactivated viral vaccines are generally safe, similar vaccines against respiratory syncytial virus and measles were withdrawn as they exacerbated the disease.



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