BENGALURU: Regulatory approval for two new vaccines a day after the announcement on administering ‘precautionary doses’ of Covid-19 vaccines for healthcare workers, frontline workers and people over 60 has intensified the battle against the pandemic.
Presently, with no data available in India on which vaccine should be used for a third dose (booster dose), certain questions are emerging on the efficacy of various vaccine combinations and how the administration of these boosters should be carried out.
The immunity from the primary vaccine series might decline over a few weeks to a few months, which might warrant a booster dose. Dr Saikiran S, Consultant Physician at People Tree Hospitals, says, “The efficacy of the booster to Omicron is still in question. Nevertheless, the possibility of additional protection and boosting immunity is a possibility. In the western world, there is some data regarding vaccine efficacy to Omicron. We need to have additional information and research into this.”
Heterologous or homologous for a booster?
Unfortunately, there is no Indian data on both of these, says Dr Gagandeep Kang, India’s top virologist and microbiologist at CMC Vellore. However, considering the data available from some countries, virologists in India say that mixing vaccines (heterologous regimen) and using a vaccine other than the primary regimen for the third dose has stimulated a stronger immune response.
“At the moment though, there is no data for the third dose in India. But we do have studies done in the US, called Cov-Boost study, from the University of Southampton, which gauged the immune response generated in individuals who had already received two doses of the Astrazeneca (Covishield) vaccine by either a booster dose of the same vaccine or the Novavax (known in India as Covovax vaccine),” explained Dr Kang.
Explaining further, Dr Chandrakant Lahariya, an expert on vaccines, public policy and health systems, says though both modes have been found to be immunologically effective, limited global evidence has shown that mixing of vaccines can, indeed, stimulate a stronger immune response.
“Any vaccine, if given to a person, and if their immune response has not been completely maxed up, you will see an increase in antibodies. The government is using immunogenicity data as a substitute for efficacy, but we don’t really know if that is the case,” Dr Kang says.
Which vaccine would make the best precautionary dose?
Apart from the six Covid-19 vaccines approved for use in India (Serum Institute’s Covishield, Bharat Biotech’s Covaxin, Russia-made Sputnik-V, Zydus Cadila’s ZyCoV-D and the US made Moderna and Johnson & Johnson) the Centre recently approved two more vaccines — Corbevax, manufactured by Biological E, and Covovax by Serum Institute of India (SII). With this, there are eight vaccines in India’s vaccine basket.
However, while considering a precautionary booster dose, the majority of the population has been vaccinated with Covishield, some with Covaxin and a very minimum population with Sputnik-V.
“From the Cov-Boost study, we know that the third dose of Covishield increased the Geometric Mean Ratio (GMR) by 3.25 times while the booster dose of Covovax increased the same by eight times,” Dr Kang pointed out.This essentially means that for those who have taken Covishield vaccine, the government could look at either giving the same vaccine as third dose or give Covovax for a better response, explains Dr Lahariya.
Experts say it is unfortunate that there is no such study for Covaxin or Corbevax yet.
“There is no data on Covaxin boosting, unless it’s with the company! However, there are a few studies with other inactivated virus vaccines, but it is a bit problematic to use the data directly. If we consider the use of that data, then Astrazeneca (Covishield) on top of two doses of inactivated virus vaccine still gives a very good boost,” explained Dr Kang.
Agreeing with this, Dr Lahariya cites the example of the Indian Council of Medical Research’s (ICMR’s) own “interesting study on 18 people who got mixed primary schedule (which) also showed that Covaxin, followed by Covishield, gave better antibody response.”
So, the option for Covaxin takers is either the third dose of the same vaccine or Covishield could be an option. However, Dr Kang says her colleague at CMC Vellore, Dr Winsley Rose, is conducting a study on this and the results should be out by February.
Availability of vaccines
Karnataka’s recent Census projections for 2021-2022 has shown that the state has to vaccinate 31,75,000 teenagers aged 15-17 years with Covaxin and 15,31,600 citizens aged above 60 years with comorbidities with the precautionary dose.“Considering that the availability of Covaxin so far in Karnataka has been good, we shouldn’t be having any issues for the teenage population. However, if the government decides to go cross-vaccinating all those who have taken Covishield with Covaxin, then supply issues will be a major one,” explained a health department official.
However, Dr Kang explains that the big advantage of Covovax and Corbevax is that they can be made very easily in huge quantities. “About 100 to 200 million does of these can be produced every month. In India, if you are giving only boosters in addition to doing primary immunisation for people, there’s more than enough to use them as boosters. Also, Serum Institute says that they are already doing 250 million doses of Covishield.”Corbevax’s manufacturer Biological E has released a statement indicating that it is aiming at a monthly production of 100 million shots by February.
Time gap
The government’s decision of a nine month gap between the second and the third precautionary dose is reportedly based on results from five small-scale studies which showed that immune responses decline considerably in the 7th-9th month after double vaccination. However, experts feel this long gap could also be for feasibility reasons.But, Dr Kang says, the longer gap may be better, but again there is no data yet on this.
Tread cautiously: Experts
While the WHO has been repeatedly warning of vaccine inequality, doctors in Karnataka also say that the “Three for us and none for you” concept won’t work in defeating the pandemic. Dr Vishal Rao, Member, Genomic Surveillance Committee, and Dean, Centre for Academic Research, HCG Cancer Centre, says, “We need to understand that we live in a global village. Nobody is safe until everybody is safe. We have to vaccinate everyone.”
Meanwhile, Dr Kang expressed concern saying, “Vaccines are not like drugs. You can’t pop one every day.” She explains that although it is now evident that older people need a booster, jumping too quickly for the whole population “may be doing a disservice to people. If you are treading with the concept that let’s keep antibodies high always and boost it with vaccines every six months, when do you stop?” she asks.
She says, “This is the right time when our scientific community needs to come together to design studies that will allow us to decide a better policy based on data on local evidence.”