Delhi saw 15,730 dengue cases in 2015, by far the capital’s worst outbreak in recent years. In 2010, when the last major outbreak took place, over 6,000 cases had been reported. The nearest comparable case load was in 1996 with over 10,200 cases. The year also saw a high mortality rate for dengue with 38 deaths compared to only eight deaths in 2010. In 1996, 423 deaths from dengue were recorded.
These numbers were personified by the tragic news in September, of a couple in South Delhi committing suicide by jumping off the roof of their house together, after their six year son died of dengue. The boy, who was already critical by the time his parents tried to have him admitted to hospital, was allegedly denied admission in four private hospitals because they were out of beds or could not manage his deteriorating condition.The case sparked outrage over hospitals refusing admission to patients diagnosed or suspected of dengue—especially the poorer people.
The state government, did not anticipated the scale of the outbreak and ill-prepared to manage the crisis. The government machinery went into an over drive trying to create additional resources within days. Around 1,000 beds were purchased within four days, disaster wards were opened for dengue patients, and private hospitals were directed to create additional beds in corridors and wards outside — wherever space could be created. Wards were opened up in porta cabins within days. Hospitals awaiting licenses were authorised to start admitting only dengue patients.
Meanwhile, microbiological tests in September to understand the virus at AIIMS and the National Centre for Disease Control (NCDC) showed that it had struck Delhi in its most virulent form. This baffled scientists because according to the cycle of the virus it should have come in a milder form, though the numbers were expected to be high. Type 2 and 4 strains of the virus, both strong strains emerged as the dominant strains in 2015. Type 4 is especially rare in the capital. Barring stray cases in 2003, the type 4 strain of the virus had never been isolated in Delhi.
The dengue virus has four serotypes, depending on the proteins — called antigens — that make it. They are related, but have minor differences in DNA. Each type has its characteristic symptoms Type 1, the classic dengue fever, and Type 3, which causes high grade fever without shock, are identified as relatively mild serotypes. The severe strains are Type 4, which leads to fever with shock, and Type 2, which causes thrombocytopenia or a drop in platelets, haemorrhagic fever, organ failure and Dengue Shock Syndrome (DSS). Globally, Type 2 has been identified as the most common cause of Dengue Haemorrhagic Fever (DHF).
Since 1960, Types 1 and 3 have been the most common in Delhi. In the 1996 outbreak, the severe Type 2 virus was identified as the most common type. The virus continued to circulate for next few years but in significantly reduced numbers. In 2003, when a sharp rise was reported again with over 2,000 cases, Type 3, a mild strain, emerged as the most common type.
In 2010, a new strain, Type 1, emerged as the prevalent strain. In the immediately following years, Type 1 remained common. The next sharp spike came in 2013 — with over 5,500 cases and the stronger Type 2 as the most common strain. This year, both Type 2, and the rare and strong Type 4 have emerged as the dominant strains.
Scientists say that for viruses like dengue which have different antigenic types, a population develops immunity to a certain type every few years. Slowly, fewer cases of that type are seen. The virus remains in circulation, but affects fewer people. During this time, other types of the virus emerge but take some time to grow and make their presence felt. Since the population has not been exposed to the new types of the virus, they affect more people, and so the number of patients shoots up. Effectively then, every spike in dengue usually indicates a change in the dominant serotype — which also means that the symptoms of the disease see a significant shift.
In December, Mexico cleared the world’s first dengue vaccine manufactured by French pharmaceutical Sanofi Pasteur, after phase three trials in Asia and Latin America. This week, the Philippines also cleared the vaccine. There are caveats: the vaccine is approved only for those aged between 9-45 years, thus exempting two large sections the virus afflicts very commonly – children and the elderly who are likely to have other existing conditions and hence are prone to more complications of the virus. So far, the vaccine is approved only for endemic populations — those living in countries where the virus is known to be in circulation. So tourists visiting these countries cannot so far, take the vaccine.
It is a tetravalent vaccine, which means it offers protection against all four virus serotypes. Though the company is still to file for approvals in India and many experts caution against its efficacy, the WHO and governments of all endemic countries, including India have said periodically that a vaccine will amplify dengue control efforts, especially since no treatment for the virus is still available, and management is only based on symptoms.
India has been trying to manufacture its own indigenous dengue vaccine since 2007, which has completed its trials on mice with positive results. Scientits will be trying it on primates in the next phase. The research for the project is being conducted under Dr. Navin Khanna of International Centre for Genetic Engineering and Biotechnology (ICGEB). Scientists estimate it to take another five years before this vaccine can reach human trials.