nOPV2: A new hope in battle against vaccine-derived polio

The new vaccine will reduce the risk of vaccine-derived outbreaks of polio.

A child being given pulse polio drops, in Palakkad, Kerala, in 2022. | Photo Credit: MUSTAFAH KK

Live oral poliovirus vaccines (OPVs) are known to be safe and effective and have led to the elimination of polio from most of the world. However, in rare circumstances the attenuated viruses in OPVs can mutate and reacquire virulence. This can result in vaccine-associated paralytic polio and lead to the emergence of circulating vaccine-derived polioviruses (cVDPVs), particularly in settings with poor immunisation coverage.

Following the global eradication of wild-type 2 poliovirus, the risk of vaccine-associated paralytic polio and cVDPVs led to the global withdrawal of the type 2 virus from OPVs for routine use in April 2016, and only bivalent OPVs containing types 1 and 3 began to be used. Since this switch, only trivalent inactivated poliovirus vaccines (IPVs) have beenused in routine immunisation campaigns against type 2 poliovirus.

However, though the routine use of live type 2 OPVs (OPV2) was stopped, type 2 cVDPV (cVDPV2) outbreaks occurred in many countries, resulting in cases of acute flaccid paralysis, which represent the majority of such polio cases worldwide.

Now, according to a report published in The Lancet, the results of a recent phase 2 trial in Bangladesh of a novel oral poliovirus vaccine type 2 (nOPV2) was found to be well tolerated among newborns and resulted in nearly all infants developing neutralising antibodies after two doses. The trial, conducted between September 21, 2020, and August 16, 2021, randomised 334 infants to receive either two doses of nOPV2 or a placebo, administered at age 0 to 3 days and at 4 weeks.

The authors noted that the novel vaccine is more genetically stable than Sabin OPV2s and, therefore, reduces the risk of cVDPV2, to which unvaccinated newborns are especially vulnerable. The nOPV2 vaccine had been studied in clinical trials of infants, children, adolescents, and adults who had already been vaccinated against polio. The current trial is the first to evaluate nOPV2 in newborns who have not received poliovirus vaccine. At birth, nearly all the infants, 93 per cent, had seroprotective antibodies against type 2 poliovirus that had transferred from their mothers. By eight weeks, only 56 per cent of the placebo group still had seroprotective maternal antibodies. In the nOPV2 group, the 90 per cent seroconversion rate from two doses of the vaccine resulted in 99 per cent of the infants having seroprotective antibodies at week 8. The authors reported that the vaccine was well-tolerated, causing mild or moderate adverse events. The amounts of virus the infants shed were low; at week 6, only 14 per cent of infants shed measurable virus, minimising the risk of a cVDPV2 outbreak.

More than 450 million doses of the vaccine have already been distributed to control cVDPV2 outbreaks, according to The Lancet paper.